I know its gonna be in the pain in the ass but I would call them and go for an exchange for the product promised or refund. Print out the website ad, and if they refuse threaten to challenge the charges on your credit card. You can also inform them that a FX 5900 Ultra with 128mb of RAM does not exist while you are at it. Then file a complaint with the Better Business Bureau: www.bbb.org
It's not so much that you got a card that is terribly slower than the one you thought you were getting - for the most part, where the 5900 non-ultra is going to chug, the 5900 ultra is going to chug, too; although there are exceptions. What it's really about is you got swindled. A 5900 non-ultra shouldn't cost you more than $200 max. Plus, I know I wouldn't be happy knowing I got robbed of $100+ and didn't follow up on it until I got that money back.
If you can get a refund, go for that and grab either a 5900SE/XT for $175, as it really isn't much slower than the 5900 Ultra, or wait 2 weeks or so for the 6800 non-ultra to come out.
Thats my 2 cents.