I have a Dell Dimension 9150. It still runs fairly well, but I dabble in PC games and it really can barely run modern games on the lowest setting. Here are the specs
I've been meaning to put a few hundred dollars into to give it a bit more power and to learn a lot about computers (since I've only installed RAM before), but looking at some old threads, there are a few issues;
a) DDR2 RAM sticks are basically the same price as DDR3. I need to install two in parallel, and it feels counter intuitive to invest in dated tech that's the same price bracket. Thing is, Dimension 9150 can't handle RRD3s.
b) The 9150 motherboard can't take dual cores but only slightly more powerful Pentium 4s, which is moot so...
c) I can't update the graphics card much since I can't upgrade the CPU
So, I thought I would buy a decent computer case ($50), cheap motherboard and CPU, RRD3 sticks, and lower end graphics card, then moving everything to the new case.
Problem is power (only got a 400W PSU), the fact that most of the other components are probably out of date as well, and, in the end, I'll be spending more than $300 just to update an old computer that isn't worth reinventing.
Problem with PC's that old is the second you upgrade something, the rest of it has to follow. New CPU will require a new motherboard, which will require DDR3 RAM. A new GPU will require a beefier power supply. Likely you will need a new HDD as IDE isn't on new mobo's, then a new copy of Windows for that. Optical as well, again IDE.
So if you try to upgrade it, the only thing that will be left is the case. You might as well just build a new PC outright and use the old one as a HTPC or something.