There was an 8800GT floating around for $208 on dell.com over the weekend. It’s not worth it at all to get an 8600GT, its a bad investment period.
HD3850 is a nice choice just under 200 right now, but I’m sure the 3870’s will be around $200 in a month…
Also whats the model number of your psu? Rosewill make terrible psus… except for maybe one or two in the 750w range. If you get me that info I will let you know if it will have ripple problems. You don’t want a psu to be the downfall of a great system - frying your mobo, vid card, ram, etc. etc.
http://shop4.outpost.com/product/5449449?site=sr:SEARCH:MAIN_RSLT_PG
or
http://accessories.us.dell.com/sna/products/Games/productdetail.aspx?c=us&l=en&s=dhs&cs=29&sku=A1350995