Got some RAM from a friend...

Solution
It would give you 8gb of ram but they would all be running at the PC2-4200 speed, which would be slower. It would also be difficult to get them to run stable together.
Dear, ddr2 RAMs give tough times when it comes to handling their timings and speed manually. RAM you care curious of are out of stock. This kit is PC2-4200 with timings of 3-3-3-8. Your current RAM is PC2-6400 with timings of 5-5-5-18. You may end up having compatibility issue or stuck up with manual adjustment in multiplier or voltage regulation for overclocking/underclocking RAMs so that they can run together.

Save this hassle and better buy Corsair XMS2 2x2GB PC2-6400.