What do you expect? You have given no details that would help give a valid suggestion. Motherboard, CPU, RAM, monitor (yes the resolution required will be a factor), PSU, case and OS. Allthese things need to be considered. Its not as simple as ticking a few boxes and hping things work. Some components have compatability issues, so like I said, give more.
Also, graphics wise, what are you looking to run? Games? Photoshop? Video? For photoshop and video its not really a decent graphics card you need but lots of ram. You're looking at mid-range performace cards on that budget which suggests gaming. You can run decent games with a budget card if you have a monitor with a low resolution. FS? Budget card again, less than a $100. Bang for a buck won't happen, ATi 5 series is out but its not that big an increase over the 4 series so not worth splashing out for. I'd avoid nvidia. IMO find a 1gb 4850 or 4870. 4870 is the higher end of the card.
To put this in perspective, I run an intel C2D @ 2Ghz on an Asus motherboard with 2gb ram. My PSU is a high end brand to cope with that and is rated to 560w. The graphics card is an ATi 4850 (512mb GRAM) and the monitor is a Xerox TFT at 1280 x 1024 (not a particularly high res but a nice monitor and no need to replace right now). I can play FS on that with Steam, XFire, Vent, IRC and AV running in the background and still achieve an FPS of 90 or more.
You can buy a high end graphics card, but if you're relying on a low end PSU that is bundled with the case and only rated to 450w then its wasted money. Also, buying a modern performance card and trying to fit to a miniATX case means it wont fit. Do your research otherwise don't complain when I only give one link 
Bite my shiny metal Tie Fighter!
Closed Alpha Test & Deth..
