If you will upgrade your current system:
1) Consider upgrading your ram to 6gb or even 8gb. That will cost $50 -$100.
With lots of multitasking going on, you can't have too much ram.
This, of course assumes that you have a 64 bit OS.
Here is a study on 6gb vs. 3gb:
http://www.corsair.com/_appnotes/AN811_Gaming_Performance%20Analysis_6GB_vs_3GB.PDF
2) Your x2-6000 is rated 1580 on this passmark cpu chart:
http://www.cpubenchmark.net/common_cpus.html
upgrading to a X4-940(passmark=3560) will cost you about $150.
You can market your old cpu for perhaps $50 on e-bay.
total for cpu +6gb = $150. Future upgradeability is minimal.
2) If you save and go to a X4-965(passmark=4227), you will pay about $150 for 6gb of ddr3 ram, $190 for the cpu, and about $140 for a good mobo with two pci-e slots.
Total =$480, less $50 for your old 4gb of ddr2, and $50 for your old cpu. The net cost is $380. future upgradeability is possible, but not announced.
3) For a i7-920(passmark=5449) upgrade, the cpu is $290, the mobo and ram are about the same, total= $580 less $100 for old parts net=$480. future upgrade to gulftown; past that is unknown.
Upgrading the current cpu is clearly the most cost efficient route. But... does that take you where you want for long enough.
Options 2 & 3 are about equal in performance per dollar. Does either one fit your budget, and get you where you want to be?
Two years out, who knows what good choices will be. I suggest meeting your near term needs now and don't plan too far out.