I think SktA is ever reaching it's limits and retirement soinvesting $100+ in a new mobo (like nForce2) is prob not the way to go unless you really need good quality integrated components. If you want a new mobo I'd suggest the SiS as low cost, very stable alternatives although they are unlikely to handle 333FSB and are very lacking in tweaking or o/c'ing options. Otherwise do consider VIA, KT333 esp with 8235 south are very good and cheap, esp if you ensure they can take 333FSB CPUs too ... otherwise KT400 is a very safe bet and always comes with 8235, 333FSB and other benefits. AMD760 is pretty old but they were very stable mobos so if you can stick the likes of the XP2400+ or XP2600+ I'd definitely stick with it even if you 'only' put an XP2000+ to XP2200+ in there for the mo.
512MB is more than enough for almost everybody and PC2100 DDR is excellent for 266FSB CPUs so there's no need at all to upgrade there, in fact even the Athlon 1.4ghz is close to XP1600+ perf which is no bad thing at all BUT considering the low cost of the XP's upgrading wouldn't hurt at all. Then there's your GF3, getting old but still more than capable of handling ALL current games very well indeed.
So in summary your PC is fine, you could go for the likes of an XP2000+ because they're so cheap, but do check what your mobo can handle. Another good option is something like the GF4TI as they start from about $100 but at stock speed the 4200 isn't a whole lot faster than a GF3TI500 esp on an Athlon @ 1.4ghz so perhaps a $200ish Rad9500PRO would be the option but again it would be slightly wasteful if you keep your Athlon 1.4ghz for anything more than the short term. I wouldn't think a mobo upgrade nor RAM upgrade would be at all cost effective unless you currently lack the ability to run XP's. Here's 2 good links showing diff gfx cards with diff AthlonXP speeds ...
AnAndTech with many games
AnAndTech focusing on UT2003
And for a rough guide here's 3Dmark:
ATHLON 1.4ghz:
GF3 - 8100, 119, 44, 120, 64, 124, 57, 58
GF4TI4200 - 10500, 147, 46, 196, 108, 135, 61, 71
Rad9500/9700 - 11000, 137, 43, 213, 113, 134, 56, 96
ATHLONXP2000+:
GF3 - 9000, 137, 55, 133, 74, 140, 67, 51
GF4TI4200 - 12200, 178, 55, 225, 122, 151, 68, 87
Rad9500/9700 - 13700, 170, 58, 267, 141, 169, 74, 109
Obviously do bear in mind you don't buy a Rad9500PRO - 9700PRO to play games without AA nor AF at 1024x768x32! It would seem that a 4200 would be a decent upgrade and an XP CPU would be good too so long as your mobo can take one.