I'd recommend completely uninstalling anything and everything having to do with a video card on your computer. Shut off, take out your old card, and put your new one in (or just reset if you already have your new one in). Reinstall drivers\control panels\etc, and you're good to go.
OK, from what I understand, you have an old computer with Half Life 2 on it, and it works. Now you have a new computer, and it won't let you log in. This is how security works. Steam won't let two computers have identical accounts. Basic copy protection?
Anyways, I think there is an...
No, but I'm sure you could make a pretty nice one using just the basic windows theme and some color modification. or you could even get adventurous and make your own theme...
I dunno, I have mine set at 512MB, which is half my RAM. I usually think of memory sizes in powers of two, so eh. I'm thinking too logically for the situation :P