If the RAM you are adding is the same voltage and timings it should/might work. If it is not the same speed as the 1GB stick all the RAM will run at the speed of the slowest stick. I don't believe your mobo will be a dual-channel board, but if it is don't be surprised if dual channel won't/can't be configured.
Worth trying. If the system fails to boot or is unstable, pull out the two 512MB sticks and it should return to normal. I ran an old AMD board (ASUS A7N8XDeluxe) this way for a long time.
Also, the math is off: you will have a total of 2GB's.: W7 64bit prefers and runs better on 4GB's.