Just for the record, ATI cards will still work under Linux. However, I have heard horror stories of people having bad experiences with getting certain things to work on Linux with an ATI card. Nvidia has much better official drivers for Linux, found here. Hell, if you run Ubuntu you can choose...
You mentioned in your original post that you may eventually dual boot Linux. If you do plan on doing this, I highly recommend going with an Nvidia card. They have much, much better Linux support.
I have the money, I just don't want to go crazy and buy a bunch of games and then find out I made a calculating error somewhere and now I can't buy groceries or pay rent or something. These aren't my first purchases this sale. :D
Though I'm probably just being overcautious.
Yeah, but you have to experience the original too. Just like I hope nobody would base their opinions on Star Wars by watching only the prequels and not the original trilogy. Not that I'm implying Black Mesa will be bad, I bet that it'll be good and I can't wait for it. ;)
You haven't played the Half-Life series?! I ORDER you to get Half Life Source. I GUARANTEE that you will enjoy it. Then get Half-Life 2, which is the single greatest PC shooter that has ever been made, hands down.