i'm shure it would speed up programs on your computer, although some people that use kazza limit their upload speed....but in all ...it would increase the speed some.
the other factor will be your end of the bandwith.
Faster Harddrive coul do that; 7200rpm will do it great. Faster Internet could help - less cluttered Hard Drive - Defragmentation very often as download is dreadful on that.
But Mr. Mixx is right; Kazaa Users will Limit their uploads sometimes; some will just have a Modem.
RAM will help too; overclocking by .2Ghz won't help much but may help your computer generally if you get it right. Cache you won't easily change yourself; if you can at all.