Video card upgrade

for 600 dollars?

grab two GTX460's in SLI. you'll have cash over, and it will be a massive performance gain.

if you want a single card option, (due to power supply or mobo constraints) I'd simply suggest a 5850. though for a similar price it offers much lower performance.

LINK: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

What's your mobo, PSU, and screen resolution?
I agree with welshmousepk, 2 260s will be a killer for ~ $400, but if you're determined to spend the whole $600, go for 2 5850s. If your mobo and PSU can handle them that is!

If you're going the Fermi route, I'd suggest the 1GB 256-bit card.

http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...

The 460 is the best bang for the buck for the time. Get a couple of them in SLI and you're good to go for a while. It'll give you better performance than the 5850 imho.

Keep in mind that 2 cards means twice the power consumption

If you want to consider the 480, here's a few:

http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...

Though I dunno if its a smart idea to get the 480's in SLI....not due to compatibility isses (there won't be any!), but simply because these two monsters will consume power from the depths of hell!

For the 5850's, thy these:

http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...
http://www.newegg.com/Product/NewProduct.aspx?Item=N82E...

Conclusion:
The 460's in SLI should do the trick.

Yes, if you motherboard is SLI two GTX 460s are the way to go.
If it is crossfire get an HD5850(or two I guess) and OC the hell out of it. Get one that allows for voltage modification for a better OC(usually 1ghz+.) At that point it will give you performance similar to a stock GTX 480 but for $150 less while also using much less power.