Well.. u`r existing mobo setup is 1G shorter than mine. Chances are, the 1G u`ve got is 2x512, and u`d prodefinably want 2x1G.
U ~could~ see if any of the guyz have 2x1G hanging around they want to sell u. I`d rather that than buy new atm, as I feel prices DDR wize are on an upward scale. *bear this point in mind*
It would however depend on how long u feel u`d have to w8 to buy a new mobo & cpu. 2x1G might be a nice stop-gap.
On the third hand, can u squeeze much more juice out of u`r 3800x2 ? How many hertzles u running it at atm, they`re invariable nice clockers (within obvious AMD limitations). I`ve gotten 2350 out of mine, I think some1 else on here ran it @ 2700/2750, and I believe we`re both stock or atleast nothing more than a hybrid air cool. Price a nice h/s ? or again see if some1 has one.
If the stop gap is going much over £120-£150, I think u`d need to lean toward the new mobo/cpu suggestion, coz it would indeed be some wastage. But, as I mentioned, some guyz on here might have some stuff to sell off.