computer shopping

[MrForbes]

I'm shopping for computer parts to upgrade/build my son's machine, looking to be able to run inventor 10 (and 11?) reasonably well, on a budget.

My guess is that a full featured cpu running at least 2.6 g, 1 g ram, and a half decent video card should do it.

Are there any specific things I should look for? hyperthreading, ddr2, agp/pci-E, etc?

I generally try to get a lot of bang for the buck, meaning I'm cheap and won't spend much more than $100 on a cpu, or $50 or so on a mobo, etc.

I think a bigger monitor is in the plan too, is 18" lcd sufficient?

thanks for any experienced suggestions!