I'm guessing you have a socket A (462) mobo and processor, right? Well, if you are going to get a new socket mobo, say 939, you would most likely need new ram as your ram is PC2700 and if not all, most 939's only support PC3200 ram.
I would recommend the following:
http://www.newegg.com/Product/Product.asp?Item=N82E16813130491
MSI K8N Neo4-F 85$
Either stick with the 3000+ or get the 3200 for like 10$ more
http://www.newegg.com/Product/Product.asp?Item=N82E16819103535
AMD 64 939skt. 3200+ 152$
What you like to do will determine what you would want in a graphis card. If you are a gamer/graphics designer or think you may become one later on, I suggest an Nvidia 6600GT
http://www.newegg.com/Product/Product.asp?Item=N82E16814150098
130$ after rebate Not particularly in love with XFX, but it's cheap.
or if you're on a budget, something like this (I'm not too sure, so ask around first)
http://www.newegg.com/Product/Product.asp?Item=N82E16814122226
Leadtek 6200TC
60$
RAM
http://www.newegg.com/Product/Product.asp?Item=N82E16820145440
Simple, do not get if you are wanting to overclock..
80$
=
337 - 447$
You may also want a new case if you have a name brand hp, emachine, dell, etc. system.
A new PSU may also be needed.
My recommendation - keep that computer in tact, possibly sell it, and build a new computer.
