I reckon that the basic PC isnt that bad. I havent seen anyone mix and match memory like you have; I was always told that normally causes issues, but their notes suggest otherwise.. weird.
Personally, I would suggest whip out the RAM you have and put in two identical 1GB modules of DDR PC4000 - if it supports Dual Channel Memory, make sure you get the 2Gb dual channel kit versions.
If budget isnt an issue, then a faster Core 2 Duo processor if the motherboard supports it would be a good investment. If the motherboard cant handle it, which i dont think it can then a new one is an option, but its getting pricey now and you maybe better of in that instance, getting a new PC!