as soon as you see a party in clapham you HAVE to go there...
nah but the big institutions are Fabric, Matter and T-bar (afaik). Then Aqualounge or sommet I think its called... basically just check resident advisor.
If you are just in it for a quiet one I'd recommend InSpiral bar in Camden (some really good nights, although you have to put up with ecological beer and yoga-heads)
Brick Lane/Shoreditch for midweek drinking & socialising.
Clubwise: Fabric is not worth the hassle (you'll spend 2 hours getting past the front doors). Haven't tried the reopening of The Den (AKA The End), might be worth a try if you've never attended the venue.
Record shops: get down to Berwick Street just off of Regent Street W1, couple of decent 2nd hand shops in Camden to.