new i7 rig with some concerns, looking for comment

Hi,

After some research on core i7 and PC retailers, I create a configuration in here. Being a student, I mainly use Maya, Photoshop, Nuke, Aftereffects, for 3D lighting & rendering works.

Specifications:
Chassis Model: Antec 900
Processor: Intel Core i7 920 2.66GHz
Motherboard: ASUS P6T Deluxe (Intel X58 Chipset)
System Memory: 6GB DDR3 1600MHz OCZ Platinum Edition
Power Supply: 650W Corsair TX (Dual SLI Compatible)
OS: I will install XP 64-bit by myself.
Hard Drive Set 1: Operating System: 500GB Western Digital (16MB Cache) (7200 RPM) (SATA)
Optical Drive 1: DVD±R/RW/CD-R/RW (DVD Writer 20x / CD-Writer 48x)
Internet Access: High Speed Network Port (Supports High-Speed Cable / DSL / Network Connections)
Video Card: 1x NVIDIA GeForce 9800GT 512MB (Includes PhysX Technology)

I may try to Overclock it, but now now.
Several concerns here:
1. Case: If I want to add a Thermalright Ultra 120E for overclocking later, will the case get
enough room for that?
2. RAM: is it OK to stay with OCZ? Or I should get the Corsair for overclocking?
3. Power Supply: Is 650W overkill if I plan to overclock and put 2 more internal HD and 4 more external HD, plus some accessories like printer? (the only lower option is 550W on their site)

Thanks!