Don't know if you've gotten it to work by now, but did you boot up to the BIOS and tell the computer to boot from the new drive? It sounds like such a stupid thing in this day and age, but the boot order might have gotten screwed up and the new HD needs to be the primary boot-up method. I would also recommend removing the old drive (or at least, unplug it from the motherboard) if you haven't done so, because as soon as your computer detects doubles of every program it'll more than likely freak out in some way.
I hope it all works out. I did the same thing with my PC in the middle of last year, and nothing has improved performance better. Memory has never done much for me, and video cards make all the games run smoother and prettier, but there's nothing quite like booting up my long-suffering hobby-built PC (the thing has been patched like Frankenstein since 2001 or so, upgraded as needed over the years) and still being shocked at how quickly it starts. Can't wait until the larger size ones are available more cheaply so we can all start slapping magnets on our PCs without fear, as we've all wanted to do.
Oh, on a serious note, once it IS running, there are a couple of tweaks you should do to add longevity to the drive. Make sure defragging is turned off (they don't need it, and defragging actually shortens the disk's lifespan), as well as a couple of other things like indexing. There's all kinds of guides out there that tell you how.