Where you would see the most performance upgrade would be the CPU and the GPU.
Hello - if the money is there, where would I see the most value in upgrading purely for GW2 performance?
In this case, any i5 would be a significant upgrade over the Core 2 Duo. You would see less of a difference going from one i5 to the other. That is to say, you would see a big difference by going to an i5, but you won't notice much difference between an i5-4460 and an i5-4690.
On the GPU front, an R9 270X would handle GW2 at nearly max graphics. An R9 280[X] would handle it easily. An R9 290[X] would be overkill at 1920x1200. So, you can pick a card to fit your budget.
I'd suggest starting with an i5-4460 and an R9 280/280x/285 and then adjusting things to fit your budget. Get a case you like and get a good quality 500-550 watt power supply such as a Corsair CX500 or similar. It's mostly just important that the motherboard has the slots you desire - which for most people means 4 DIMM slots and the usual bunch of USB and PCIe slots. Don't worry about upgradeability of the motherboard - by the time you want to upgrade, you'll most likely be going to a new processor that uses a new socket and new RAM, and therefore requires a new motherboard as well. So just get one that works now.