Run 1 stick on its own usually slot 1
2 paired sticks same capacity, timings, etc in 1 & 3
Some mobos wont accept or handle three sticks, Assus A8SLI & PW5's.
4 paired sticks ok
So shuv the two 512's in 1 & 3 slots, originals move to 2 & 4 totalling 1.5Gb.
You are lucky mine was on offer at £270 5 months ago, so only had the two 1gb sticks PC8500.
Had a lot of misfortune with unbranded stuff, usually dont boot, even had it on this one knocked up over Christmas and many others.
Only buy new top name branded stuff, different if machine is a couple of years old and strapped for cash though, there again cheap enough new.