Your CPU should be fine, but your slow RAM and the PCI 1.0 will slow you down some. However, if you drop the money into the GPU now you will see a performance increase, and you can simply move it over to a new PC that you can build next year. The 670s won't be quite as top of the line next year, but they should still be able to handle any current games on good settings, and if need be you can buy a second one and SLI them.
tl;dr - Buy the GPU if you want for an immediate increase. Build a new computer next year when you've got the budget for it and reuse the GPU.