To determine your system specs; goto Start --> Run, Type
dxdiag and click OK. Your CPU (processor), Memory and DirectX version is listed on the
System tab. Your video card specifcations can be found under the
Display tab under Name and Chip Type. Your soundcard is listed under the
Sound tab in the Name field. Some other (free) tools you can use are CPUz and PC Wizard (both available from
http://www.cpuid.com)
To be honest, even though you probably can get a 20GB drive (and it would be an improvement), those drives are antiquated and space per buck, they are a bad choice. The "financially ideal" HDD would be in the 80-160GB range depending on your budget
To play any recent games at anything deemed acceptable/good, you're gonna have to get a much newer video subsystem. I suspect you have a onboard videochip (because the 4MB cards werent very long lived) ... in either case you shoulld definitely make this a priority
System memory 128 Mb GCC SDRAM
I'm not familiar with GCC SDRAM and in either case, 128MB, while "doable" isnt the greatest for optimal gaming. I sell computers and in most cases, 512MB is the optimal "minimum".
Of the two (video and memory), odds are the video card is more crucial to gameplay and RAM will affect loadtimes and overall system performance. You may be able to overclock the CPU since P3s dont make very much heat however I'll need to know more information before I can comment on that