Its ok to make a game that runs fine on your PC but If you never gave what it would look like on another PC a second thaught, your in trouble.

Compatability is alot more than how much memory your game takes up, or how fast the PC has to be to play it, Its about Timing. To be a truely good game creator you must have timeing that runs acurate to a milli second across all PCs with enough memory and CPU power to play it.
When I was a newbie, I made the fatal mistake of spending ages getting the timing right on my PC and doing it in such a sloppy way that it would fall apart on a faster or slower PC.

As I havn't had time to spend ages experimenting with different methods of putting games together and testing them on other PCs I cant give you any advice but this: If you have more than 1 PC, test it on another 1, If you have a friend that wants to help, ask him to test it.

And Finally if youve got to thst stage were your game is 99% done and all thats left is to put on the final shine and give it 1 last testing, don't rush to release it. I've been in that position and thaught, great a game that I can put on the DC, I'll just take a few screen shots, put it in a zip file, upload it and I can have it on the net in 10 minutes. Then I find a fatal floor in the game that could of been easily detected.

Sorry about the longness but compatibility and timing really matter or you'll look like a fool to the player.