Hi newcomer and welcome to the Tom's hardware forum.
1- If you want a RAM that run at 1200MHz, you need buy a RAM 1200MHz or OC your current RAM until 1200MHz
2- You can install your trascend 2GB 800MHz and 2GB 1200HMz (any other manufacturer), and you only see your RAM running at 800MHz, if you want see the 1200MHz, you need OC the 800MHz or buy 4GB 1200MHz
3- I'm very sure that you won't see a big difference with 800MHz or 1200MHz and with the price of the 1200MHz RAM, you can buy an AMD Athlon II X2 620, in other words, Don't waste your money, your option 1200MHz isn't a good option price/performance.
well thanks Friend
good suggestion !