Personally I build a new box with all new stuff. If your existing box has age and stuff is failing, is it worth having another old component start dying in your new rig?
I do leave mine on, running site performance checks and log file dl and analysis and backups and crap like that all night though. So, my main machines run 24/7/365.
I use 64 bit, haven't had any software issues for stuff I run, though I do remember not installing something (don't remember what atm) that wasn't compatible with 64.
Intel procs, personal preference. I've always used them. AMD is better bang for the buck if you're a gamer. Intel is better at multi core performance.
Get as much memory as you want to afford and fits in the mb.
I would go with a ssd for os/apps and a bigger one for data if needed.
Older graphics cards can be iffy if you run them 24/7 and they have their own fans. Again, I like to have all new stuff on a new build and would consider this card:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814139055&cm_re=jaton-_-14-139-055-_-Product