Your best bet right now would be the Saphire Radeon 9600pro 128MB.
Don't buy in to the 256MB trap unless your buying more the high end cards because they can often be slower than their 128MB counterparts.
www.newegg.com should give you some of the best prices.
I'd suggest this for $128 shipped. SAPPHIRE ATI RADEON 9600PRO Video Card, 128MB DDR, 128-bit, DVI/TV-Out, 8X AGP, Model "ATLANTIS RADEON 9600PRO" -RETAIL
http://www.newegg.com/app/ViewProduc...102-291&depa=0
And if you are in a hurry to get this PC together, by all means, get your vid card now, but if your willing to wait a month or so more a new round of cards are comming out. That should drop the prices on some of the high end cards now and provide you with new, possibly better options for your PC.
If you are even more budget minded you can try a refurb for around $100. $108 will even get you a 9800SE.
http://www.newegg.com/app/ViewProduc...tby=14&order=1