if you're wanting to jump up to around 1ghz most likely you will need a new motherboard. it's hard to tell you exactly what to look for......one site may have the same product 30 bucks cheaper then another. the advice i would give you is to find 9 or 10 different sites and compare prices and user reviews. keep an eye out for mobo+cpu combos....sometimes you can save money if you buy them as a package instead of seperately. you will also need to check and see if your old memory will be compatible with your new mobo.....that may slim down your selection of mobo's somewhat. Also make sure that if you get a new agp video card that is compatible with the mobo as well.
i've done most of my shopping at
http://www.newegg.com and of as yet have not had a single problem. a lot of their products have free shipping (you really have to play close attention to shipping costs...some online retailers try to be really sneaky about it)
I recently got a ATI Radeon 9000 there for 50 bucks. as long as you do a little research you should be ok......if you end up feeling like you got ripped off when all is said and done you chalk it up to experience. it happens to all of us.
here's my major components to give you an idea as well:
AMD XP 2000+
ECS k7s5a mobo
384 pc2100 DDR memory
Chaintec 64mb GF4 MX440
Maxtor 40gb hard drive
Lite-On 16x DVD
