Hello everyone!
For sometime I have been contemplating building my own system. I am an Engineer and feel up to the task; however, if anyone has some advice or a critique of my build I would greatly appreciate the help.

Antec 900 case

ZALMAN ZM750-HP 750W Power Supply

Intel Core 2 Quad Q9450 Yorkfield 2.66GHz 12MB L2 Processor

GIGABYTE GA-EP45T-DS3R Motherboard

GIGABYTE GV-RX385512H Radeon HD 3850 512MB 256-bit GDDR3 PCI Express 2.0 x16 Video card
Considering the SAPPHIRE 100242TXSR Radeon Toxic HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16

ZALMAN CNPS 9300AT SuperFlo CPU Cooler + Arctic Silver 5 Thermal Compound

3xWestern Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive (in RAID5)

EITHER CORSAIR XMS3 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666)
OR OCZ Reaper HPC 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800)

Pioneer 20X DVD±R DVD Burner Black

Microsoft Windows Vista Home Premium SP1 64-bit

Any tips tricks or opinions are welcomed!

And just a few questions to you OCing folks out there,
My CPU (8x multi) will run at 2.66 GHz at a core FSB of 333 MHz right?
If I bump that up to 400MHz I should hit 3.2 GHz correct?
If I bump up to 400 using the 1333 ram will it jump up to 1600 also?

Also, if i was using the 1600 what would it jump to, the MoBo says it can handle 1600MHz FSB and 1900 on the ram, so would it hit 1900? or would it have been running at 1333 until I upped the FSB and then be running at 1600

Yes to 1st 3 o/c questions. Last one: you will need to change cpu/ram ratio to make use of 1600mhz ram.

i.e. FSB 333mhz x 2 = 666mhz DDR2. By default, any ram will run at 666mhz in that config.

IMO, you shouldn't bother with DDR3 now. Swap mobo out for GA-EP45-DS3R and ram for DDR2. You won't see a difference between 800mhz ram & 1066mhz ram in everyday use. Only in benchmarks. Swap the cpu cooler out for Xigmatek HDT-S1283 + backplate kit. Use stock TIM. If you like, swap AS5 out for MX-2.

He is right, if your fsb is up to 1600 mhz ddr2 800 would be plenty enough. Otherwise you will not note a difference between ddr2 and ddr3 as of now. If you plan on gaming get a videocard nvidia gtx280 or wait untill the 4870x2 comes out. Hard drive is good, case too I guess. If you plan on gaming get a dual core such as E8400 it is easy overclockable to 4ghz with a decent heatsink. If you want multitasking, go quad.

Ok thanks for the help on the ram math.
I'll go with that Mobo or one like it and the DDR-800

I think I will also go with the E8400 instead.

Though if I get the Radeon HD 3850 or 4850 now, could I Crossfire it with the 4870x2 in a year or so?

Only the series. i.e. HD 4850 + HD 4870 = ok. I don't know about HD 3800s.