Did you reboot after installing the nvidia packages? You can't load the nvidia module while nouveau is also loaded.

Sure! Many times :)
I installed arch many times before, and never had such a problem.

I found an interesting thing. Described problem exists only on i686 platform, on x86_64 everything is OK. I figured that nouveau also fails to initialize the card (on i686), on x86_64 both (nouveau and nvidia) works OK.
Strange thing is, that on Live environment (I start Arch Installer by iPXE netboot) nouveau fails on both i686 and x86_64 (I see by dmesg "unknown chipset" by nouveau), but after installing, on i686 it still fails, but on x86_64 it works (and nvidia too). Tried this four or five times (installing i686 and right after x86_64, and after i686 and again...). Another strange thing is, that when all works (x86_64) lspci verbose shows a "Subsystem" section for my graphics card (Compal ... bla bla bla), but on i686 it does not show this. Does it matters? I include all dmesgs (for both nouveau and nvidia modules installed separately) and both i686 and x86_64, and lspcis from this configurations. Hope someone will help. (I want to use i686 flavor of Arch - sigh :/ )

I dig in deeper and found more interesting things.
I tested this on other distros (Debian):
1. Lenny 5.0.10 (Old Old Stable)- kernel 2.6.26 - nvidia 173.14.09
2. Squeeze 6.0.7 (Old Stable) - kernel 2.6.32 (with nouveau) - nvidia 195.36.31
3. Wheezy 7.1.0 (Stable) - kernel 3.2.0-4-686-pae (with nouveau) - nvidia 304.88
4. Jessie (Testing) - kernel 3.9.8-1-686-pae (with nouveau) - nvidia 304.88
5. Sid - kernel 3.10.3-1-686-pae (with nouveau) - nvidia 304.88
6. Jessie (Testing) - kernel 3.9.8-1-686-pae (with nouveau) - nvidia 319.32 from nvidia.com (as a *.run package)
Tried 7. Sid - kernel 3.10.3-1-686-pae (with nouveau) - nvidia 319.32 (and319.17) from nvidia.com (as a *.run package) but it wouldn't build, even with "kernel 3.10" patches for it, dunno why (always one hunk (#9) have failed).

On every one of this configuration nvidia module initialized my card without problems. Where nouveau was available it also initialized my card OK. I was thinking that the kernel 3.10 is source of the problem, because:
1) it breaks compatibility for building nvidia drivers (patches needed - see devtalk.nvidia and various distros boards)
2) for me, it somehow breaks compatibility for detecting my graphics card (the "Subsystem" section for this card in lspci -k disapears in kernel 3.10 which (I think) causes the nouveau driver and nvidia driver not to recognize my card ("unknown chipset" / "NVRM: installed in this system is not supported by the 319.32").

But when I installed Debian Testing (Jessie) and upgraded it to Sid (Kernel 3.9 got upgraded to 3.10) - nouveau started OK - initialized my card ("Subsystem" section is visible) - so WTF?
When I installed Debian Testing (Jessie)(kernel 3.9), installed nvidia-driver (304.88) and then upgraded to Sid (kernel 3.10), it started but without loaded nvidia module (no "subsystem" - in dmesg: NVRM: installed in this system is not supported by the 304.88). But when I cold restart (full poweroff and start) it loads normally. When I instaled Testing (Jessie) upgraded to Sid and then installed nvidia-driver it loaded upon warm restart immediately.

Next what I tried was Arch i686 but with nvidia 304.88 (by changing url and md5sum in PKGBUILD from nvidia-full-beta-all from AUR):
- I couldn't do that, because the "patch" failed, instead of this i installed nvidia-304xx form pacman - result: NVRM: installed in this system is not supported by the 304.88
WTF^2? In any other distro my card is supported by 304.88 and even lower. Whats more, on Arch x86_64 this scenario works 100% (install clean OS, install nvidia-304xx, nvidia gets loaded OK). WTF^3?

The other thing is - when I start Arch x86 Live (netboot) it ALWAYS fails to load nouveau (no "Subsystem" section), but when I start Arch x86_64 (netboot) it initializes nouveau for 1 start on ~15 starts. I cant find the "trigger", which causes nouveau to start.

Now I am confused, if it is kernel 3.10 fault, or if it is Arch's kernel and/or Arch's nvidia module fault. The truth is: on windows the card is 100% functional, on Debian Stable (Wheezy), Debian Testing (Jessie), Debian Sid it is 100% functional, unless you decide to install a very unstable and untested driver from experimental (even never than in Sid) on kernel 3.10 (which is only on Sid and never [even not in regular "Testing (Jessie)"].

Unfortunately I can fully confirm these issues, however with a newer nVidia Card in a recent Lenovo Y500 Notebook with 2x Geforce 650M (2GB). Until Kernel 3.9.X I had no issues at all, but wanted to upgrade to 3.10 since the on-board ethernet will fully work with this kernel version, as I've been told in the AUR (dkms-alx) package. Unfortunately, the upgrade broke my whole system by simply boosting up the graphic card during bootup and then no video output at all. My system works perfectly well with Windows 7 and Debian Sid, so its not a hardware problem either.

Downgrading my Kernel to 3.9.9 + older nVidia Package, that I had on my desktop machine, resolved the issues for me at least for a while now. Theres definitely something odd with Kernel 3.10 + nVidia for notebooks, since there are quite a few people with the same problem in the forums and on the nVidia Support Webpage.

Please try 3.10.5 with latest nvidia from testing repository.

Checked (i686):
Nouveau NOT working with 3.10.5-1.
Nvidia NOT working (319.32-6).
All problems the same as described.

Checked (x64):
Nouveau not working with 3.10.5-1
Nvidia not working (319.32.6)

Same as Bart (obi-wan) wrote for me, too.

Please try 325.15 from testing

Tested, effect the same (NOT working) 325.15-1 on i686.

Same here for x64 :-(

This here is a support thread in nVidias Support Forums:

https://devtalk.nvidia.com/default/topic/567297/linux/linux-3-10-driver-crash/

Somehow maybe not only a arch related problem.