What did you end up doing and how does it work?
I'm also currently investigating this, and many searches later I haven't found much definitive.
I can add a little info: the +12V is 16A:http://i52.tinypic.com/m7s19i.png
Dell says that the x16 slot can support 25W max, which I think is the ultimate arbiter, though finding max draw for cards is not easy.http://support.dell.com/support/edocs/systems/dimc521/en/SM_EN/specs.htm
Another thread here said that the 8400S would work, though with those you'll commonly see "Minimum of a 350 Watt power supply. (Minimum recommended power supply with +12 Volt current rating of 18 Amps.)"http://www.evga.com/products/moreinfo.asp?pn=01G-P3-1302-LR
Interestingly, the much higher-end Galaxy GeForce 9600 GT LP definitely works, since I've seen multiple posts to that effect, but who's actually selling it now? It seems to be quasi-discontinued despite no mention on the vendor's site. Also, confusingly, the vendor site it needs 300W with 12V @ 26A, so you would think it would have no shot on the C521, yet people say it does.http://www.galaxytechus.com/usa/productview.aspx?id=79