Can I use an old Pentium 4 desktop for mining if I upgrade it with a good GPU? I know that the most important thing for mining is a GPU. I read some stuff about it but I never read a specific answer to this simple question.

I have some old desktop computer. Pentium 4 (~1GHz) processor and (maybe 256-)512 MBs or RAM.
Can I use this computer as a base for a simple mining machine if I can just put in a decent GPU??
I'm just strolling the internet for second-hand GPUs and asking friends, but I cannot afford to invest too much in it, so I'd like to be able to use that desktop for my setup.
With a decent enough GPU (or GPUs), your machine will be a fine miner. The only spec that ACTUALLY matters for mining is the GPU. RAM, CPU, etc don't have much of an impact.