I have a Asus P5N-E motherboard. My video card is a 8600GT I want some more graphics performance, but I am unsure whether to get a second 8600GT and run them in SLi, or just get a better video card to replace my current one. My processor is a dual-core Pentium, (E2180). Is it worth getting a better video card, or a second card for SLi, or should I get a better processor, as well? If so, I will just wait until the price of quad core processors comes down dramatically.

SLi (and the mobo's that support SLi on Intel platforms at least) sucks UNLESS you run a really REALLY high resolution, have THE fastest card on the planet or get the cards really REALLY cheap.

2x 8600GT's in SLi are going to be pretty crap. Don't bother.

Get something like a new ATi 4850 - as your technically minded i'll leave it up to you to see if it's ok (i.e. PSU etc).

you should try out ati HD 4870 XOC tuned by diamond,looks and sounds promising(im not amd fan btw)
here:http://www.techpowerup.com/index.php?64301

Quote from e2mustang :

you should try out ati HD 4870 XOC tuned by diamond,looks and sounds promising(im not amd fan btw)

Maybe if it weren't $300, I would. Or if I had a job, which I don't. Money doesn't grow on trees!

4850, best bang for buck right now. Great card by all acounts.

The problem of SLI (I had one) is that the average and maximum fps boost a lot, but the minimum fps (which are the most important) are still very weak.
Besides, don´t forget the microstuttering.
SLI is fine (even for low resolutions if the cards aren´t high end) but one card is always better, so you have to avoid dual systems whenever possible.