You can buy an AMD Phenom II X2 555 Black Edition + GIGABYTE MA785GM-US2H and keep your DDR2 modules. With that mobo you will be able to try to unlock the aditional 2 cores to become a Phenom X4 555.
I would suggest you to buy the following heatsink: COOLER MASTER HYPER 212 PLUS.
All this for 180€ aproximately.
Take in mind that with that mobo you won't be able to do cf because it only has one PCIex16 slot. For doing cf you should buy the Gigabyte GA-MA790X-UD3P (is DDR2 too) for 25€ more.