I have no hard data but from looking at review sites, here's what I *think*.
You would likely get performance boosts from either a cpu or vid upgrade, but the boost you'd get from BOTH would be even larger. The 1600+ cpu will hold you back, and the GF3 is just plain OLD. With prices where they are and considering what hardware you have, If I were you I would definately spend a couple hundred bucks because you can get a noticeable benefit for not too much money.
Not only would you get a performance boost from a new vid card, you could also get better image quality by being able to turn on FSAA and ansio filtering.
Consider getting a multiplier-unlocked mobile barton 2500+ and then overclocking it by increasing the multiplier. Or if you're not comfortable overclocking, any of the faster barton processors would be fine. Since your mobo supports barton processors, you have a cost-effective upgrade path through replacing only the cpu, but I wouldn't spend more than $150ish on the cpu upgrade. You can get a barton 3000 for about that, but make sure you have a nice heatsink/fan unit to keep it cool. I like the thermalright SLK series (SLK-800 is what I have) because you can put a quiet 80mm fan on it and get better cooling than a noiser high performance 60mm fan, but that's personal preference.
Even a sub-$100 barton 2500 would probably be noticeable and would save you up to $50 depending on where you got it, giving you some more coin to drop on a video card. Either way, I bet you will notice almost any upgrade even if you spend a total of under $350. But I'd go with both a better cpu and vid card because I'm fairly sure you'd be pleased with the results.
512 meg memory should be fine, so don't worry about that unless you have a specific need for more ram.
IMHO
