Looking to build a new rig over the next couple of weeks. Let me know what you think of it.

Case - Xion II Black ATX Mid-Tower Case with Clear Side Front USB and Audio Ports and 450-Watt Power Supply

Power Supply - OCZ / GameXStream / 700-Watt / ATX / 120mm Fan / SATA Ready / SLI Ready / EPS / Active PFC / Power Supply

CPU - Intel Core 2 Quad Q6600 Processor HH80562PH0568M - 2.40GHz, 8MB Cache, 1066MHz FSB, Kentsfield, Quad-Core, OEM, Socket 775, Processor

Motherboard - EVGA nforce 780i SLI Motherboard - A1 Version, NVIDIA nForce 780i SLI, Socket 775, ATX, Audio, PCI Express 2.0, Dual Gigabit LAN, S/PDIF, USB 2.0, Firewire, Serial ATA, RAID

RAM - Centon Dual Channel 4096MB PC6400 DDR2 800MHz Memory (2 x 2048MB)

Sound Card - Creative Labs Sound Blaster X-Fi XtremeGamer PCI Sound Card

Hard Drive - Maxtor OneTouch III 750GB External Hard Drive - 7200 RPM, 16MB, USB 2.0

Video Card - EVGA GeForce 8800 GT Video Card - 512MB DDR3, PCI Express 2.0, SLI Ready, (Dual Link) Dual DVI, HDTV, Video Card

Optical Drive - Lite-On LH-20A1S SuperAllwrite SATA DVD Burner - 20x DVD±R Burn, 16x DVD±R Read, 8x DVD+RW, 6x DVD-RW, 8x DVD±R DL, 12x DVD-RAM, 48x32x CD-R/RW

Operating System - Microsoft Windows Vista Ultimate 64 Bit

Is this all I need to build a rig or am I missing something?
Will it all fit good in the case I listed?
Is it all compatible with each other?
External Hard Drive will be only hard drive, is that ok?
Overall is this rig decent for gaming?