I have a Gigabyte H55M-S2H motherboard with a core i7 870. I'm currently running Windows7 64-Bit and I am interested in upgrading my RAM to 8GB. Can anyone suggest a suitable brand with a high mhz that will work with this motherboard.
Kingston HyperX 2x2GB KHX1600C7D3K2/4GX 1600MHz kit will do fine. If you're not overclocking heavily and seriously benchmarking, anything more than 1333MHz isn't even noticeable in usage. I took it as "default" that you already own 4GB RAM, so I only suggested a 4GB kit to be added on.
No need to replace a dual channel kit, a new kit can just be added. Computer won't complain, the RAM will just run at the lower kit's speed and for dual channel to function you have to have the same chips in the opposite channels (kit 1 on channel 1 and kit 2 on channel 2). And Kingston is more reliable than Corsair, TBH...