If you are just looking to upgrade a couple components i would start with upgrading that E7500 to a Quad Core like a Q6600/6700/6800 or even better, a Q9450/9550/9650.
After that, the next best step is to overclock to 3.6 or as close as you can.
My reasoning for the above is that while games run fine on dual cores, they tent to do better with 3 cores. 4 cores usually proves to be at the same performance level as 3 in about 90%+ games.
as for that Video card, it is still a decent card but if you were going to upgrade it, i would only do so if you've upgraded said CPU above and something along the lines of a 5850 or better would serve you best.
You could also consider corssfire for that 4890 but i am not entirely sure how Rift scales with Crossfire, so you would want to look that up. Also, i would not crossfire in another 4890 unless you upgraded the CPU as stated above.
hope this makes sense.