Blizzard's put up the System Requirements for Burning Crusade (2nd to last question on the FAQ), so if your current box is chugging along, you might want to check them out. The other reason I decided to post this (it's right up there on the WoW site, after all-- why come here when you can see it there, right?) is because I wanted to tell you that I just got my new PC case, so my new monster rig is now just a week or so away!
OK, fine, I'll give you a little more service than that. Just for you, readers, I reached up on my shelf and pulled down my WoW box to compare the old system requirements to the new BC ones. Interestingly, the expansion does call for a little more RAM (512mb, 1gb recommended) than the original game does (256mb). The original box doesn't say they require a Directx sound card, but apparently the expansion makes that distinction for whatever reason. On the Mac side, while the original game required OS X 10.3.5 or newer, the expansion calls for 10.3.9 or newer. I don't run WoW on my Mac, but I believe all the 10.3.x updates came free with System Update anyway, so I'm sure you'll be fine.
And even if you don't match up to the recommended specs, I wouldn't worry too much about it-- Blizzard's games are very forgiving on the hardware (not so much when you're running a full AV or MC, but when you're soloing in Hinterlands for sure). Then again, if you do need new RAM, now's the time to start looking.