
I see you're currently using an Athlon 1.2ghz which it would be worth upgrading next, esp considering how cheap AthlonXP1800+ to XP2000+ are. Anyway on to the cards.

Here's a quick run down of the current nVidia offerings:
GF2: Lousy image quality (IQ), GF2TI is a good 50% faster than the GF2MX.
GF3: Slightly better IQ but are essentialy nothing more than adding DX8 hw and a whole lot faster in 3D inc AA & AF. TI200 is a fair bit slower than TI500 but tends to o/c VERY well.
GF4MX: Suped up GF2 cards seriously improving 3D perf, IQ, dual display, hw DVD playback and slightly enhance TVout/VIVO too. 3D-wise MX420 is a fair bit faster than GF2MX and is very near GF2GTS perf. MX440 is faster than GF2TI and GF2Ultra (esp GF4MX440-8X) while GF4MX460 is faster again but still no where near GF3 3D perf and still completely lack DX8 hw.
GF4TI: Suped up GF3 cards seriously improving 3D perf (esp on faster CPUs), IQ, dual display and slightly enhance AA and TVout/VIVO too. 4200 isn't a whole lot faster than GF3TI500 but it o/c's to GF4TI4400 levels which is significantly faster. TI4400 is about 15% faster than a 4200 and 4600 is about 10% faster than 4400.

Then on to the Radeon cards:
Rad7500: GF2GTS/TI 3D perf but with the same sort of enhancements GF4MX have (IQ, DD, DVD etc).
Rad8500: GF3TI500 3D perf inc DX8 hw but again offer GF4MX style enhancements (IQ, DD, DVD etc). 8500LE is about 10% slower than 8500.
Rad9000: Quite a lot slower than Rad8500 but are cheaper to make. They enhance IQ and DVD playback further but 8500 are certainly better cards. Rad9000 is 20% slower than Rad9000PRO which in turn is 15% slower than Rad8500LE (10% slower than Rad8500).
Rad9700: By far the best card currently available incorporating DX9 and awesome 3D, AA, AF etc. But it does cost!

So without a doubt I'd say 4200 is your card. Without getting too deep go for the 128MB version. Brand matters very little so cheapest is often best. The only things to watch out for is the odd manu who only supply 1 output port which kills DD and the manus known to skimp on some of their 4200 cards are AOpen, EVGA and the standard Gainward cards. The Albatron 4200 P 128MB from
Newegg seem very good and very popular. Rad8500LE-128MB is great if you need to spend less and at Athlon 1.2ghz isn't much slower than a GF4TI4200, it's a fine alternative.