Can I use 2gb and 4gb modules of DDR3 in my mobo at the same time?

Aite then, this is toms hardware, so im lookin for a simple answer to an even simpler question.
This is my system specs- CPU: core-i7-860 @ 2.8GHz, Graphics Card: Ati Radeon Sapphire HD5850 1GB GDDR5,OS: Win-7 Ultimate 32Bit build 7600, Mobo: Gigabyte-H55M-S2H, RAM:- 4GB DDR3 (2x2GB 1333MHz FSB Memory Module) Display: Samsung SyncMaster P2230 Lavender Series HD LCD (1920x1080 res, 60hz refresh rate)

As you can see from the specs, everything is quite dandy if i may say so, but whenever im playing an intensive game or multitasking like listening to songs and browsing 50/100 tabs in IE or Firefox, my pc freezes up, hangs, and on rare occasions, restarts itself. So im planning to get me some more ram,my mobo has 4 slots for DDR3 RAMs,currently im using two of'em with 2GB's ,so that makes it 4GB,but since Win7 32 bit only reads about 3.7GB, im gona upgrade to Win7 64 bit, now my Q to you guys is, recommend me a good fail-proof memory module option, i know that u gotta use the same fsb and mem module at the same time,but since i got 2 slots of 2gb's, can i install

1. just one more 4GB DDR3 with the same fsb and mhz to make it 8GB?
2. or do i need to install a 2GB again to make it 6GB?
3. or do i need to install 2 more 2GB to make it 8GB total?
4. or do i need to install two more 4GB's to make it a total of 12GB?

so these are the options that i have thought of, and im confused, so please tell me which options are viable for me according to my mobo and specs, or gimme other options that u know work for sure. just tell me what works or what type of mem module shud i buy. plus also lemme know if i can make 1033/1066/1666 mhz fsb rams to go work with my 1333 ones, or do i must i rly nid the same fsb mhz

n oh ya,thanx alot in advance for helping me to choose where to put my bucks.   ciao