My previous system was:

Q6600, stock speed
8GB, DDR2 at 800mhz
4870X2 Video (2GB)
Asus Motherboard
Raptor 10k rpm HD.

Wasn't happy with the performance, could lose follow in Dalaran with all toons mounted on ground mounts. Follow frequently broke in Alterac Valley or Wintergrasp (if mounted).
Instances and 5-boxing elsewhere were fine, but had a lot of the settings down.



Decided to upgrade a couple of summers ago.
Went with:

i7 920, stock speed
12GB, DDR3 at 1600mhz.
4870X2 Video (2GB)
Asus P6T Motherboard
Raptor 10k rpm HD.

Had the computer store do the upgrade for me; I don't like mounting motherboards, having cracked one during installation quite a few years ago.
They goofed up and put Vista 32-bit on, instead of 64-bit.
Even with only 2GB of ram usable (video card was using 2GB), the performance increase was massive; aside from crossing phase points, it was impossible to break follow on epic flyers unless the lead toon had Crusader Aura on (and even then it was difficult). On ground mounts, without getting them stuck on corners it was impossible to break follow.
And this was with only 2GB of the ram.

Got the OS fixed.
Had some kind of a conflict between the Video Card, and one of the upgrades (Processor, Motherboard or Ram).
One or two slave windows would randomly freeze (requiring a reboot); the freeze could occur at any point, but always did when taking a gryphon anywhere.
Had this issue with both Keyclone and IS Boxer, so pretty sure it was hardware related.
Eventually (through changing one part at a time), I isolated the issue to the video card.
Downgraded to a GTX275, which has been much more then sufficient for 5-boxing Warcraft (the 275 benchmarks comparable to a 1GB 4870 (not a 4870X2)).

As an aside loaned the motherboard, video card, processor and ram to a friend. The video card is working great for him.