I am currently looking to upgrade my 1.5 year old dektop. Unfortunatley the only way I can upgrade it further is with the purchase of a new motherboard and processsor. Currently I own a dell 2400 dektop. The motherboard sucks because it has been stripped down to where it only has 3 pci slots and the agp slot is missing. The case (tower) is a mid size case I believe. Would it be possible for me to replace the motherboard in this case (tower) or should I buy a new one? I am a college student so I really dont have the cash for a new system. I plan to upgrade the rest of the computer later on.
Anyways I am looking to spend no more than 400 dollars on the motherboard and processor. I would prefer an amd processor. Any suggestions to what I should purchase would also be helpful. The system is mainly used as a gaming machine and upgrades should reflect this.

Here are my full system specs:
Pentium 4 2.2 ghz with 400 mhz frontside bus
1 gig ram @ 266 mhz
radeon 9250 with 256 video memory
soundblaster live 24 bit
The motherboard is whatever came stock with the system.