Need new processor + vid card = get new comp?

Would I be better off getting a whole new comp, or just buying a new processor and vid card?

Also, would I need to buy a new motherboard?

Hey I have a similar question to OP but I actually have the pertinent information for it.

So I wanted to get some opinion's on the bottleneck/s in this system. I'm trying to play Oblivion which is ok until I get into a fight and everything slows down so much that I die.

Sempron 2400 overclocked at 1780, 512MB GeForce7600GS overclocked at 518/784 core/memory (recommended overclock by the nvidia utility), 1280MB DDR266. MB is ASRock K7VM3 if you're wondering.

I don't want to buy anymore parts, but do you think I could overclock the CPU more and would that be the main problem? Or is single-core CPU and DDR just so outdated that it's hopeless?

Also the BIOS has a setting for AGP(yeah it's 4X AGP :() aperture max is 256MB does that mean I'm only getting half of the video memory??

Also I try and defragment my drives cause I think it always slows down when it's accessing files, but it always goes for like 10 minutes and says "some files could not be defragmented" and gives up.

Any of these questions answered would be appreciated.