Currently, I'm running at about 30 FPS with medium-high settings. What would I have to buy/replace to get that above 60 (I play without VSynch) at all times?
You need a new CPU + motherboard + RAM.
Since it's a Dell, consequently, a new copy of Windows 7.
GTX 550 Ti should easily do ~40 FPS with all ultra (shadows @ high), with some multi-sampling.
This assumes you are talking about city / raid FPS.
Your GPU is OK, but definitely not something that'll get you 60+ in cities and raids. If you absolutely gotta have 60+, you also have to replace:
Graphics card, and power supply, and perhaps the computer case.
Due to the limitation in WoW engine, you will not see constant 60+ FPS everywhere in all times in cities and raids.
You want the following:
B75 motherboard (cheapest is $60ish, ensure microATX)
8GB (2 x 4GB) DDR3-1600, 1.5v RAM (about $48)
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