This game doesn't really require really high end gear if you are willing to sacrifice some eye candy---it can run quite well on hardware that was released back as far as 2008/2009 if you are willing to trim the graphics settings. Even trimmed down, at 1920/1080, it still looks pretty good. You can save a lot by going with standard drives over SSD, it mostly will impact your zone loading times for the most part. I would stick to the 7200 RPM's though...5400 RPM's can be quieter and save energy and all, but that may be pushing the limits of your patience when it comes to startup and zone load times.

Some key things to make sure you don't skimp on too much:

Processor
--because of the sheer number of players online (coupled with variety in models), you will need some decent horsepower. High end CPU's can get quite pricey, so this is a common area for people to trim their costs a bit---just be careful not to go too far down the list with your options. High end Dual Cores will work fine for this game, but others take more advantage of the Quad Cores if you can swing it in your budget. You can compare scores at the Passmark site to narrow down your choices if you aren't sure on some CPU's. You can get by with scores in the 2000-3000 range (mid to high end chart) if you are willing to accept low frame rates during areas of high congestion, but you really want to shoot higher, like around 4k+ if you can (on the High-End Chart) if this is somewhere you are looking to cut costs a little. This also ties in with a future-proofing angle, as with the higher end CPU's you'll be in a slot design that will allow you more scalability for a CPU upgrade later if needed.

Memory
DDR3 memory comes in different speeds, and your transfer rate can make a considerable difference. Your Graphics card is going to have blazing fast memory on board, and the PCI-E bus is capable of feeding it data at a wicked pace as well. 16,000 speed RAM with their potential 32GB+/second transfer rate sounds great, but you will be paying for it. You should be able to score a decent price on some 12,800 memory. This can still keep you above a 20GB/sec memory transfer rate, which will match quite well with most graphics cards with DDR3 memory (typically around the 20-30GB/sec rates for 128bit GDDR3). You can feel the difference if you go lower--especially if your graphics card winds up having 256bit DDR5 RAM on board (these can run upwards of 64GB/sec). Remember, you have to continually feed data to that screaming demon of a graphics card to maintain a good fill rate on your screen.

Graphics Card
This can be a tricky one. Again, Passmark has some nice charts for comparing models--can look up a lot of cards directly with the search link to the lower right corner. The game is still running on DX9, so at the moment you can go cheap in this department in the short term, and then upgrade later to something beefier. If you opt to cut some corners here--make sure to not go too far back. There are a few key things to keep in mind when shopping to help you weed out a lot of them:

--Hardware DirectX support.
Make sure it supports DX11 in hardware. There are some really good cards out there still, but they only go to 10.1 in hardware. If you make sure to get one with full DX11 hardware support, you will stand a better chance of it working with more recently released games even though it may be an older card.
--Memory Architecture and Capacity
Obviously, 1GB would be better than 512MB. But, you may not need 2GB+ just yet... maybe if you are running massive resolutions, spanning multiple monitors and such, but 1GB should be a reasonable compromise for the price difference. The big gotcha here can be the Bus Width. Simply put, 256bit moves twice as much data per pass as 128bit. Make sure to compare the memory architecture when comparing cards. This can lead to a major let-down if you aren't paying attention. You may be in a good family of cards for the feature set, but you slip up and grab the 128-bit memory instead of the 256bit.

The www.hwcompare.com site is nice for putting two cards against each other in a side-by-side comparison of their features once you've started narrowing it down to just a handful of models you are interested in. It may have a limited list of models to compare, but it has a lot of popular cards in the list to give you a general idea to work with.