If you load too many addons and lack of RAM, it'll affect the overall perfomance, kinda jerky. Experienced it when I'm using 512mb and I got greedy using every single addons I think that's useful.

My opinions on the few most useful addons are:

A must for raiding:
KLH Threat Meter (or Omen, I heard it's not bad)
CT Raid Assist

Some other useful:
Nature Enemy Cast Bar
Gatherer
Scrolling Combat Text
Trinketmenu
Decursive (so you can yell over the mic)
X-Perl
Atlasloot
Titan Panel
Auctioneer
Damage Meter
Atlas
Mobinfo
Itemrack
Bagnon
Monkeyquest
Telo's quickloot

And a must for my main (warlock):
Necrosis
DoTimer
Fishing Buddy