I just got a BFG 7600 GT for around 220 pretty powerful card.. havn't tested out NL yet but Im sure if it runs HL2 with most everything on full at 120 fps it will run NL just fine.
Yea, it's a little more powerful than the 6800GS, it's really crippled by it's 128-bit memory interface :-
And yea, Windows XP can run on 256mb of memory, but for most setups, it's shared memory, which can reduce it down to 192mb. That's not anywhere near enough for XP, you want at least 300mb for the OS alone. So yea, 512mb should be the minimum memory requirements for XP, but then companies like Dell wouldn't be able to make their cheap ass PC's.