I don't make it down to NOVA too often and when I do it's Arlington, so I don't know a ton about the area. Check out
On Tap, which has info on some of the bars there in Alexandria. They don't update the drink specials too often so don't put too much stock in that, but the info about the type of crowd and general cost are accurate.
Also, I'd recommend taking the Metro if you're going farther than a few minute cab ride...in the DC area your fare is calculated by "zones" or some crap and they're such a ripoff.
If you'll be in town Saturday night and you've got some drinkers with you, hop on the Metro and go to My Brother's Place in the Capitol Hill area. You have to walk through a bit of a seedy area from the Metro to the bar, but they've got all you can drink beer for $12 and a lot of Catholic U students go there and stuff, so it's a young crowd.