I have a Dell desktop computer with a 256 MB Nvidia 7800 GT/Pentium 4 3 GHz/1.5 GB RAM/520 watt OCZ Powerstream PSU. I would like to bring it up to my dorm, but would have to put it in a more portable case. Because of Dell's proprietary standards, this would entail getting a new motherboard. Here are my options:

1. Keep the Pentium 4 CPU and get a new motherboard.
This would be my cheapest option.

2. Switch to AMD and get a new CPU as well as 2-4 GB new RAM. The CPU I had in mind was the Athlon X2 5000+.
Newegg.com - AMD Athlon 64 X2 5000+ Brisbane 2.6GHz 2 x 512KB L2 Cache Socket AM2 65W Dual-Core Black Edition Processor - CPUs / Processors

This is the case I'm looking to get:
Lian Li PC-V350B Micro ATX Mini Tower Computer Case - Black

Is the Pentium 4 still fast enough for gaming, or should I get the AMD? Is there a better alternative for a cheap gaming CPU? My budget is $300 or less for the CPU/Mobo/RAM together.

Why not sell your older stuff and get an Intel dual core?

Mobo Newegg.com - GIGABYTE GA-P35-DS3L LGA 775 Intel P35 ATX All Solid Capacitor Intel Motherboard - Intel Motherboards $109 +$8.25 shipping

CPU Newegg.com - Intel Core 2 Duo E7200 Wolfdale 2.53GHz 3MB L2 Cache LGA 775 65W Dual-Core Processor - CPUs / Processors $129

RAM Newegg.com - G.SKILL 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory - Desktop Memory $59.99 +$6.99 shipping

$313 or so. Assuming you have a PCI video card and PSU and such.

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