UPgrade nazies my ass.
The fact is, there's no excuse NOT to upgrade. Ram is cheap, Cpu's are cheap. Check www.pricewatch.com
You get a 1gighertz Athlon for under $70. 256 megs of ram for under $50. I mean, if this isnt cheap enough then you need a different hobby. Period.
Also, if you run q3 ok doesnt mean JK2 will run fine. Jk2 is based on the Q3:Team Arena code base, which is more demanding then the original q3.