Ho hum, your only choice is to try and pick up some cheap off ebay, otherwise its not really an economical upgrade, in fact if you find you are struggling with CPU speed etc then its really time for a new motherboard, chip and ram...possibly hdd too as they all have a massive impact on overall speed and usability.
One thing to remember though, you only get one pair of eyes, so look after them and dont skimp on old/cheap crappy monitors, get a decent TFT and save your eyes the strain.