Yeah if they outright lied about the memory speeds then you should be offered a refund. At the very least that's false advertising, and it sounds EXACTLY like the problem I had with my GF2MX card (which ran at Voodoo3 speeds). Though it does seem VERY odd that Asus would make a card like that and lie about memory speeds and such...are you SURE it's an Asus card? It's quite possible that they simply packaged a crappy generic card in an Asus box and sold it as an "Asus FX5200". I just don't want people to avoid Asus cards because some really dishonest seller lied about it.
Where did you buy the card from, if you don't mind me asking. My GF2MX just came from some random computer retailer on the internet. I'd recommend Newegg, they're the only place that I've never had a problem with (they even took back a fried motherboard after the warrenty was up!), and they have a 128 meg, 128 bit (at least I'm pretty sure) FX5200 availible for $71 shipped. Might be something to keep in mind in case your seller is willing to refund your money.