Another thing tha tyou need to consider is CPU tasking.
I have a quad core 64 bit AMD with 8 gigs of ram NVIDIA GTS450 and fast HD's (blah blah blah)
But I also use dual screen with widescreen monitors. Simply using dual screen taxes a video card severly adding a nonstandard widescreen resolution doesnt help much.
My second screen often uses IE with no less then 4-6 game related tabs, Loadstone , ZAM, YG, LS FORUMS, and some other stuff for crafting.
I noticed a noticable decrease in performance when IE is open because of all the FLASH and misc other animated Adds that appear on these sites being processed on multiple tabs my IE uses no less then 25% of my CPU's. When i exempted my game folders form antivirus scanning it helped alot.
Just in case your dual screening just wanted to let you know that when i went to one screen and didnt have any web browsers open the game ran alot smoother.
The game is so CPU intensive that even the most seemingly mundane background processes could make it apepar to lag when their CPU time spikes even a little.
If your not scared to take the side pannel off your PC you should put your hand near the video card to see if its radiating a severe amount of heat when the game does its freeze up. and also check to make sure that your video cards GPU/CPU fan is actually spinning and not obstructed by a cord or a dead bug (the term came from somewhere)
Take care not to touch a spinning fan with your ungloved fingers they can shatter your fingernails if you hit them at a bad angle and cause lots of pain use a pen to prod fans to make sure they spin at acceptable speeds.
Cornyboob Funnyname Wutai Phys 50 Conj 49 Weaver 49 LW 34 Alch 28 Pug 22 Archer 20 Glad 19 Thaum 20 GS 11 Bot 9 Miner 8 Shield 44 Maur 3