From a brief look at your hardware, your machine would most benefit from a RAM and video card upgrade. However, I suspect your current memory is the old 168-pin SDRAM. rather than the more recent 184-pin DDR1 RAM. If that's the case, then it isn't really worth upgrading the memory, as the availability is poor & cost-per-meg on the old stuff is uneconomical.

Similarly with your graphics - it's most likely an AGP interface, which is rapidly becoming obsolete and not well supported by the latest graphics chipsets. So you'd be forced into buying either older technology or a lower-performing chip, which you wouldn't be able to take with you in a future upgrade. All newer machines use PCI-e x16 slots, often in dual-card (SLI or Crossfire) configurations, which is something an upgraded card for your current motherboard couldn't later migrate towards.

I'd say your best choice is to continue to save up for a new machine - so in this case, patience is a virtue!