ATI 5870; 4GB or 8GB of RAM for increasing performance/FPS?

I'm about to purchase a gaming desktop with the ATI HD5870 Sapphire Overclocked Video Card, I was wondering if getting 8GB of RAM instead of getting 4GB clocked at DDR3-2000 would be worth it for increasing performance and fps or if its not even necessary. Im only using this PC to game. Also give me any tips... show more I'm about to purchase a gaming desktop with the ATI HD5870 Sapphire Overclocked Video Card, I was wondering if getting 8GB of RAM instead of getting 4GB clocked at DDR3-2000 would be worth it for increasing performance and fps or if its not even necessary. Im only using this PC to game. Also give me any tips and insight of the computer parts whether or not it will be good for gaming or anything else youd like to thow in. Heres my specs:

Processor ( [== Six Core ==] AMD Phenom™ II X6 1055T Six-Core CPU )
Memory ( 4 GB [2 GB X2] DDR3-2000 Memory Module )
Video Card ( ATI Radeon HD 5870 - 1GB - Sapphire Vapor-X Overclocked - Single Card )
Motherboard ( [CrossFire] ASUS M4A79XTD EVO -- AMD 790X w/ 2x PCI-E 2.0 x16 )
Power Supply ( 800 Watt -- Standard )
Primary Hard Drive ( 640 GB HARD DRIVE -- 64M Cache, 7200 RPM, 6.0Gb/s - Single Drive )
Sound Card ( Creative Labs Sound Blaster Audigy SE )
Network Card ( Intel Pro 10/100/1000 Network Card )

Thanks Guys!
Update: What do you mean getting another HDD and doing raid rather than 8 GB of ram? Thanks for your insight guys!
Update 2: Okay, It has that option Raid 0 High Performance, but that wont boost game performance/FPS, just faster loading times for windows and games correct? In that case, it would be pointless for me cause my system would already be fast right? I'm also thinking of upgrading to 8GB 1800 Memory speed. Keep the insight... show more Okay, It has that option Raid 0 High Performance, but that wont boost game performance/FPS, just faster loading times for windows and games correct? In that case, it would be pointless for me cause my system would already be fast right? I'm also thinking of upgrading to 8GB 1800 Memory speed. Keep the insight coming guys, I really appriciate your comments!
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