为什么我的显卡GTX5903GB， 玩游戏只能用到1.5GB

ppp162000

小弟不才··最近买了台电脑自带GTX590, 可是玩GTA4的时候，里面之显示有1.5GB··求大师指教， 附带电脑配置听别人说要打补丁··那个补丁在哪下载

1	Alienware Aurora-R3

1	Intel Core i7-2600K (8MB Cache) Overclocked Turbo Boost to 4.1GHz

1	16GB Dual Channel DDR3 at 1333MHz (4X4GB), 4 DIMMs

1	Alienware TactX Keyboard, US

1	No Monitor

1	Single Nvidia GTX 590

1	2TB RAID 1+0 (4 x 1TB SATA-II, 7,200 RPM, 32MB

1	Matte Stealth Black Alienware Aurora w/ 875W Multi-GPU Approved PSU

1	Alienware 19-in-1 Media Card Reader

1	Dell Resource DVD with Application Backup

1	Accessory Kit, Aurora 3, Eng

1	PC-Restore, Dim/Insp

1	DataSafe Local BackUp

1	Genuine Windows 7 Home Premium 64 bit, English, w/Media

1	Dell Support Center 3

1	Alienware TactX Mouse

1	AlienFX Color, Mars Red

1	Adobe Acrobat Reader

1	Single Drive: 24X CD/DVD burner (DVD+/-RW) w/double layer write capability

1	Roxio Creator Starter,No Media

1	Sound Blaster X-Fi Xtreme Audio

1	Alienhead 3D

1	Dell Hardware Limited Warranty, Initial Year

1	Dell Limited Hardware Warranty Plus In-Home Service After Remote Diagnosis, Initial Year

1	1 Year Limited Warranty

1	No Warranty, Year 2 and 3

1	Warranty Support, Initial Year

1	Soft Contracts Dell In-Home Hardware Agreement for Alienware

1	S and P Drop-in-Box Marcom forDHS Desktops

1	Alienware High-Performance Liquid Cooling

1	Alienhead 3D

1	Steam and Portal Factory Installed

[backcolor=rgba(255, 255, 255, 0.917969)] [tr][/tr]