Upgrading Machine

Can i upgrade my Ghz?
And shortly how and what?
meaning would i have to replace my hole motherboard?
i have upgraded everything myself except the motherboard

"My Specs"
Dell Dimension 2350 Pentium 4, 1.8ghz, (upgrade from 256 mb) to 2 gigs of ram, (upgrade from 200watts) to 400 watt power supply, (upgraded from a intergraded intel) to NVIDIA GForce 8400 GS graphics card, SP3, (upgraded from 30 gig HD) to 250 gig Hard Drive

You can buy a netbook for 300 dollars that has twice the computing power.

It seems like its just time to buy a new computer, it will be cheaper for the long haul. Though if you insist on wanting to upgrade you may be able to find a P4 3.2 Ghz that is 478 socket. I just looked on Newegg.com and they do not sell them anymore. You should be able to pick one up on ebay for 20 bucks. As I said before, its time for a whole new system.