5600U, though forget about DX9 with that card.  Still, $100 new is impressively low for that card.  Digit-Life's 3Digest shows it performs on par with a 4200 w/o AA/AF, and it'll be much faster with AA&/AF.  Not a bad buy for a DX8 card with decent AA+AF performance. 
Edit: Pricewatch, eh?  No such thing as a $100 5600U, then.  The $100 cards listed are plain 5600's (beware Gainward's liberal use of the word "Ultra!" with parts that aren't Ultra) or worse, 5600 XT's with a horrendous 64-bit memory bus (avoid at all costs).  Avoid the $120 XFX 5600U, too, as it's the old, slow 350/700MHz Ultra.  You can buy a BFG 5600Uv2 (400/400) for ~$120, though.  NewEgg sells the OEM version (base card and nothing else--no DVD player, cables, nothing) for 
$120 shipped.  CompuHQ sells the retail version (DVD player, DVI-VGA adapter) for 
$120 plus shipping.  I suppose NewEgg's card comes with a one-year NewEgg warranty.  CompuHQ's, since its retail, comes with a lifetime warranty from BFG.  I'd pay the few bucks for the latter.
But if you're getting up to $120-130, it's really almost foolish not to pony up an extra ~$20 for a 
$153 XT, or another $15 for a 5900.  The 5600U's HL2 numbers also give me pause, though I'm hoping performance will be on par with a 4200 when the game is finally released, as the 5600U matches the 4200 in most other DX8 games.
If you insist on new, NewEgg sells two Gainward 4200's, one Golden Sample (270/570MHz instead of 250/500MHz) for $89 and one regular (with a DVI-VGA adapter, whee) for $86.
Heck, I'd just grab keysplayr2003's $150 9700, if it's still available.  Or try to talk him down to $125. 
