Is it ddr2 - ddr3, what your processor and motherboard?
we need proper info if you want a proper answer:
but basically, it goes down to what OS your using.
32 bit os (windows xp 32 bit, vista 32 bit and win7 32 bit) can only use a max of about 3.5 gb, therefore your wasting your money
any 64 bit is fine and will use the full 8 gb although it's a bit overkill unless your doing encoding and such.