Which graphics card should I buy?

Have you considered ATI? a 5850 is in your price range. If you are investing for 2-4 years a DX 11 option might be smart.

Are you SLI/crossfire capable?

the 5850 may just bottleneck a X2 4600+.
To be honest, the video cards your looking at, you need at least a quad-core, maybe an AMD X3 to run w/o much bottleneck

I'd check this article out if you didn't read it
http://www.tomshardware.com/reviews/build-balanced-plat...

i'd go w/ a mainstream GT300 series when it comes out (if your tied to Nvidia) or an equivalent AMD card w/ DirectX 11

Consider upgrading your CPU along with the graphics card.

there are plenty of good cards that will fit your AM2 Socket motherboard that you can upgrade to as well. The Phenoms will work well. Perhaps an x3

It's not that the 5850 will bottleneck a X2 4600+. It's the other way round.
Anyways, +1 5850, if you find one. In your case, you gotta upgrade your CPU, too.
BTW, check if the rest of your system is compatible, specially your PSU.

if u dont wanna up ur cpu get 250 maybe 260

They all can do better at more than 1600x1200 or something like that.
If 1280x1080 is your resolution then i would say go with 250
Or if you like ati then take a look at 4850

intel in general get pair with Nvidia ................N ............amd with ATI

As you have AMD 64 X2 4600+ a rather go 4 the HD 5870 or 5850 as they has DX 11 capabilities with will be useful 4 NEXT generation of gaming...........

but as the HD 5870 N 5850 will surely bottleneck the system .............make an extreme overclocking of yur AMD 64 X2 4600+

ATTENTION!!!! extreme overclocking means that yur CPU will be boost 30% more than the stock speed of 2.4 Ghz....therefore around 2.9 and 3.0 Ghz

make sure that U have a decent trademark n a reasonable Wattage of yur PSU......

And finally make sure that U have a very good cooling system......liquid or air