So, the cheapest i could find for HP ram is the Corsair Twin 2X XMS2 2048-6400 (2gb kit)... how much difference would this be compare to most of the value ram (for example kingston DDR2 800 2x1GB)..?
I wouldn't overclock your memory as it shortens the period of time that it functions.
The bottomline is how much you want to spend. If you really want performance go with the Corsair XMS memory as it will be a little faster. If you want to save money go with the Kingston.
In my own opinion I would take the Kingston Value PC2-6400 memory; but I am also not a hardcore gamer. If you want to play the latest games take the Corsair XMS.