Replace or upgrade?

My current system is a 3.0GHz P4 w/Asus P4P800 mobo, 2x256MG DDR400 PC3200 RAM, nVidia GeForce FX5200 128MB, and SoundMax onboard sound. I'd like to update, and here's what I've come up with:

Radeon X800XT 256MB GDDR3 AGP 4X/8X $299
Core clock: 500MHz
Memory clock: 1000MHz
Memory interface: 256 bit
OpenGL: OpenGL 2.0
Pixel pipelines: 16

Radeon X850XT 256MB GDDR3 AGP 4X/8X $240
Core clock: 520MHz
Memory clock: 1080MHz
Memory interface: 256 bit
OpenGL: OpenGL 2.0
Pixel pipelines: 16

Total of 2GB Corsair 184-pin DDR400 PC3200 $164 to $202

SB Audigy X-Fi Extreme Music $110

SB Audigy X-Fi Platinum $185


So using these as a list to pick from, I'm looking at paying from $514 to $686. I'm sure that by shopping around I could find different hardware or lower prices but these are good to just give me a rough idea.


However, I think I could probably spend the same amount of money and get a new mobo (and CPU if necessary), 2GB+ of DDR2 RAM, X850XT PCI-E, and one of the Audigy cards, and not still have a 2 1/2 year old system afterwards. If an extra $100-$200 would let me replace instead of upgrade I'd definitely go for it. Anyone have any ideas as far as a good mobo w/PCI-E? Dual-core CPU? AMD vs Intel, and why? I sorta know about the differences in how AMD & Intel calculate clock speed. Would my current 450W power supply be adequate? I don't know if I'd plan on overclocking but I'd like to have a complete system that could handle, say, 4GHz. Expected use would be high-end flight sims and maybe some 1st person shooters like Counterstrike or Delta Force.