I'd avoid anything lower than the Radeon 7500. The radeon 7500 can be had for $50 bucks...I know, I just got one in the mail 2 days ago. 64 meg board and its a very capable 3D card for todays modern games.
Its no Geforce4 or Radeon 9700, but for $50 bucks its a lot of videocard.
You can save $20 bucks and get an older Rage 128 or original Radeon card with 32 megs, but then you're only saving $20 bucks and getting a VERY dated videocard...the Radeon 7500 is modern enough to keep you happy for awhile, especially if you plan on playing PC games on the box from time to time.
If you ahve a few more dollars upgrade to the Radeon 8500. On an Athlon 1.4Ghz system using a radeon 8500 ($80 bucks, got it 2 weeks ago) I've been playing Unreal Tournament 2003 at 1024x768 without any speed issues at all.
Anyways, the moral of the story is any of the athlons in the sub-$100 range are worth investing in. Best TV-out you can buy, but the other poster is right:
the PCI versions of the Radeon 7500 offer composite out, no SVideo out.
I've got my 8500 running Svideo out into a "Svideo to composite" Converter as my 27" TV doesn't have Svideo in, and I've found mame and Unreal Tournament to look just fine onscreen. Text is hard as hell to read, but otherwise things are ok.