I agree, it is time for a new build.
If you wanted, adding 4gb of ram would help your current performance while multitasking.
But, DDR2 ram is relatively expensive.
From a cpu point of view, the E8400 at stock has a passmark# of 2199. More with your oc.
A modern $220 3570K quad will be 7151 running at 3.4 stock. An oc of 4.3 is easy.
A $320 3770K with hyperthreading at stock is 9623, and again a OC of 4.3 is reasonable.
A Z77 based motherboard will be $150.
DDR3 ram is very cheap $70 for 16gb.
A real boost to your performance would be a SSD. a 120gb ssd will be <$1 per gb.
Use your old HDD for storage and backup.