You really can't find an 8800 on eBay?
I have a great ATI 4870, but I think it will only do one at that res.
Hmm.
http://cgi.ebay.com/NVIDIA-8800-GT-5...#ht_1530wt_941
There's one, lots on eBay, but says for 'second' generation Mac Pro, and your's is probably third generation?