Ive got XP and I never get "are you sure" popups in the registry (with XP) and I never use prefetch, prefetch is just a way to load programs faster (in my opinion doesn't work well)
all in all, XP is a lot easier to use than 98, it is especially better in things like drivers or stability.
If you really don't like XP, then it is up to you to stay with 98, but I do reccomend XP
first, how much are you willing to spend on the mobo/CPU?
if you go with my mobo/CPU suggestion, you shouldn't pay more than $650/$700 AUS, or about $350 US
if you want a good video card but without a big price, the Radeon 9200 would be plenty good enough. if you are a gamer, then with the 9600 Pro you can play high end games like HL2 or Doom 3 with ease. don't bother getting the 9800 Pro, it's too expensive
you know what computer shops can be like: a lot of them will try and sell you a new CD burner if your monitor screws up, or a new Motherboard and CPU if your hard drive dies on you, or something completely irrelevant to what you want or need.
If he tells you what we said is completely wrong, and/or that you should buy all the most expensive or useless stuff, start running!