I agree with Luc. and Khafan. Motherboard/CPU combo deals are pretty common. You could pick up a decent 3ghz P4 and Intel Mobo for like 250. Try Froogle.com and Pricewatch.com I know there's about 5 million other sites like that but those are the 2 I remember off hand.
I picked up Geforce 6600 GT (128mb) off of newegg.com for 190. It's not an amazing card but when you look at "bang for your buck" that card kills.
Get your motherboard up to speed and get yourself a card that can hold you over. Try and unload that 5900 on ebay or something for whatever you can get for it then save up for the card your really want.
When you're dealing with upgrades its good to start from the base and work off of that.. obviously for the bottlenecking. Otherwise you won't be happy a couple weeks down the line.
http://www.newegg.com/app/viewproduc...130-196&DEPA=1
There's a one day sale on a Geforce 6800 Ultra 256mb for 437 off of new egg if you're interested. Ends on the 22nd of this month (Feb) FREE SHIPPING