Off the top of my head...

Good RPGs with old graphics:
Fallout
Fallout 2
Planescape Torment
Baldur's Gate 1 & 2 (and expansion packs)

Good RPGs with moderate graphics:
Vampire: Bloodlines
Morrowind
Knights of the Old Republic 1 & 2
Neverwinter Nights 1, but only with the expansion packs.
Neverwinter Nights 2, expansion pack highly recommended.

Good RPGs with new graphics:
Jade Empire
The Witcher
Mass Effect (from what I've been told, haven't played it)