What can i upgrdade on my fx 7026 computer

What's your goal for upgrading?
And what is your upgrade budget?

Gateway FX7026 Desktop PC
Core 2 Quad Q9300(2.50GHz) 4GB DDR2 640GB HDD NVIDIA GeForce 8800 GT
Windows Vista Home Premium 64-bit
specs: http://support.gateway.com/s/PC/R/1015336R/1015336Rsp2.shtml
FX7020_NV.jpg
 
+1 WR2 for posting specs. Agree with asking usage, will assume gaming is the intent.

Not a bad gaming system, just needs stronger video. This PC comes with a Power Supply = 400 Watt that supports a factory overclocked 8800GT.

You can buy and install any card from this list that (1) has less than 200 watts TDP and (3) is at least 3 tiers higher than the 8800GT in the hierarchy chart on the last page of this article. http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html

Suggest you spend around $150 for this HD6850 http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html But spending more will get you more.