the FSB of K6-II 350 is 10 MHz and that of a AthlonXP 1500+ is 133 MHz (266 DDR), the performance difference between the two is immense, but for simple use you can stay with your 350. Even I use my K6-II 350 system on a older RedFox ALiAGP (that doesnt have ATA/66) and dont find any reason to upgrade!
But for simulator games and heavy multitasking you do need a hefty system, especially with more memory, 256~512 MB.
The upgrade will gratly help you, but I suggest you keep your 350 for simpler tasks as a second machine.
<font color=red>Nothing is fool-proof. Fools are Ingenious!</font color=red>