Computer's Artifacting

So... I'm running 2-sli'd GTX470s in my Antec 900 (V1) and my cards keep artifacting when I try to play games... I can't seem to figure out what the problem is whether it's my PSU (running an 850W), just not getting cool enough (they are REALLY close together with a sound card in between them), or something else (the hard drives giving out - but I'm assuming GPUs).

First thing is first... how should I test to figure out what the problem is. Second, should I look into getting liquid cooling (and if so, what should I be looking at - budgets around $500)? Third - should I upgrade to a full tower and get a higher-powered PSU (preferably cheap and reliable for both of those if necessary)?

Finally... I am running an older CPU (an AMD Athlon II x4 640) on a M4N98TD EVO board... Should I be upgrading that? I would rather not have to upgrade that as it's really pushing my budget as it is. I will post more specs if deemed necessary.

Please help!!! >_<