Depends entirely on your system dude. I will say this, going from 1680x1050 to 1920x1080 I had a noticeable difference on my system.
4 gig ddr2 800
Nvidia GTX 260 216 core OC
One thing I will tell you for sure.. You will want a graphics card with at least 768, preferably 1gig of ram onboard. 1920x1080 is kinda the turning point - I would say 1920x1080 and above, you really want a 1 gig memory graphics card.
Screens 1920 x 1080 and above can really utilize more ram on a graphics card. I would say that your 1280 x 1024 would be fine with a 512 ram card..