Thanks for the tips guys, still no luck though. Re-sat both sticks of RAM, and the machine didnt boot. I took one stick out and it booted up fine.
@Badger906 I managed to find a user manual online for my machine, which says it is upgradable to 8GB (see attached) I also went in to msconfig, the max memory box was already unticked.
On a side note, the new RAM stick specs are; DDR3 PC3-8500 1066Mhz (as far as i can see from my digging online, thats the same spec as the preinstalled RAM in the machine). Again, thanks for any tips