A lot of this has to do with your budget, what/how your are editing now (as Russ mentioned, the software) and other variables. Setting up your machine with a couple of upgrades and tweaks could be fine for you. (eSATA card to hook up a couple of external RAIDs if you are currently just editing on an external Firewire drive or single internal drive.)
An SSD drive for booting is really a great upgrade, together with throwing some more RAM in. The graphics could help, depending on the software you are running. Some software may not use the graphics card.
The one caveat is that Thunderbolt (for me) is really a great thing to have when moving a large amount of files. I am often moving 10-15 terrabytes arounds for back-up, additional drives and the rest. You can a fast eSata card and enclosure. But then if you go Thunderbolt later, you may need to move things around. (There are some Thunderbolt to eSATA devices, like those released by Firmtek, which I have used and some enclosures with Thunderbolt and eSATA connectors.)
Not sure how many cores or speed your computer is, but my Tower is older and does pretty well. Faster than my newer iMac (though not the most recent one.)
So in a nutshell, you may be able to get a ton more out of your current computer for less cost. I much prefer my Tower to my iMac, but for the Thunderbolt issue. (My Tower is 2009, 8 Core 2.93, the iMac is Quad Core 3.4 i7) Had to encode things recently and it was quicker for me to network the Tower through the iMac to the Thunderbolt drives via Ethernet to have the Tower do the encoding, then the iMac doing the encoding. (Again, some of this depends on which app, how it uses multi cores and the rest.)