Neverwinter Nights: Tested in Realworld (TM) by myself:
System 1: 2Ghz Northwood GF4 Ti 4200 / 512MB DDR 333 runs very well, occasional hickups and low fps when going over 1152 resolution
System 2: dual 2600MP 1GB reg ecc DDR 266 ATi 8500 retail 275/275: Slower, no hickups as bad as the NV in highres, but certainly not as smooth as on the NVidia card. (And I am more of an ATi then NV Fan.) I must admit that you need at least a 9600pro to get this thing to work really fluid. Even the 9800XT numbers arent that great. a 5900XT beats it easy.
In general, the game consumes ram (so 1GB dual channel does help) and needs a good vidcard. However, I doubt it is running well on those 1Ghz rigs. I played it on my dual when i still had 1800XPs, and it was slower then with my 2600. I would recommend the following:
2,4Ghz/2400XP
1GB DDR 333 dual channel
9600XT/5900XT
19 inch Monitor minimum. The text is tiny even at 1024. (yes I know about the high res text option)
heresiarh: Get another 512MB RAM and you should be fine.