It is better with 8GB i think, it’s also 16gb but later i can buy more of this and fill up all 8 slots. It is then 64GB total.
If i choose 4GBx4 and want 64gb later this mem will be useless? Correct me if I wrong.
No, it doesn't work like that. You shouldn't mix kits. You should never mix kits like that. If you want to put 64GB in, then you'll need to put a 64GB kit in, or you may have memory issues. If you want 16GB now, and 64GB later, you will still end up with 16GB left over.
Motherboard: RIVE (3602 bios)
CPU: Intel 3930K @4646MHz
OS Drive: 2 X Samsung 840 PRO (Raid 0)
Storage Drive: 2 X 1.5TB WD Caviar Black RAID 0, 2 X 3TB WD Caviar Red, Kingston V100 256GB SSD
Memory: 64GB G.SKILL Ripjaws Z (F3-12800CL10Q2-64GBZL)
GPU: Gigabyte GeForce GTX580 @795MHz - 1536MB GDDR5
PSU: OCZ ZX1250
Cooling: Phantek PH-TC14PE
OS: Windows 7 Ultimate x64. (EUFI)