PC upgrade options?

I have a :
? Soyo SY-K7VIA Athlon ATX, 4x, AGP w/Audio
o
? AMD Athalon 650Mhz CPU
o
? ATX Mid-Tower Case w/ Athalon-approved 250 watt Power Supply
o

I want to upgrade but don’t know what my options are. I do not want a new case. But I do want more speed.

– I could do a new CPU with current motherboard or get a motherboard / CPU combination.

If I go with a new motherboard and CPU which have the best reviews? ZDnet? Cnet? And where can I get them the cheapest?

Also,where can I buy the cheapest and best internal modem I can use with both Linux and Windows? Are there any that work with both Linux and Windows and support caller ID?