new cross fire gaming build: will it work?

First off, avoid Rosewill PSU's! You also chose a DDR3 motherboard with DDR2 RAM. I would recommend the following changes:

CORSAIR CMPSU-750TX 750W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS Certified Active PFC Power Supply - Retail
http://www.buy.com/prod/corsair-tx [...] 78325.html $103.74 -$20.00 MIR

GIGABYTE GA-EP45-DS3R LGA 775 Intel P45 ATX Intel Motherboard - Retail
http://www.newegg.com/Product/Prod [...] 6813128344 $126.99 -$20.00 MIR

and either this

Antec Three Hundred Black ATX Mid Tower Computer Case - Retail
http://www.newegg.com/Product/Prod [...] 6811129042 $69.95

or this

Antec Nine Hundred
http://www.buy.com/prod/antec-nine [...] 97579.html $100 shipped

Vista Home Premium should be all you need.

Microsoft Windows Vista Home Premium SP1 64-bit English for System Builders 1pk DSP OEI DVD - OEM
http://www.newegg.com/Product/Prod [...] 6832116488 $99.99

The WD 6400AAKS is a little faster and cooler running than the one you chose and costs the same.

Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive - OEM
http://www.newegg.com/Product/Prod [...] 6822136218 $79.99

Edit: If you can squeeze it in, change the motherboard to either an X38 or X48 chipset board to run your crossfire at full speed.

GIGABYTE GA-X48-DQ6 LGA 775 Intel X48 ATX Intel Motherboard - Retail
http://www.newegg.com/Product/Prod [...] 6813128331 $278.99

I would either get the first one in your list or this:

mushkin 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
http://www.newegg.com/Product/Prod [...] 6820146731 $76.99 -$20.00 MIR

It has great timings at the standard 1.8v.

What resolution are you looking to play your games at? You don't need CF if you're planning to play anything under and including 1680*1050.

Everything arrived... and after a day of setup, I'm finally at the BIOS screen!

I'm having a little trouble installing Vista Home Premium (64bit) with the DVD that I ordered.

After it says: boot from CD/DVD
I get this error:
Disk Boot Error: Insert System Disk and Press Enter

The disk is already in, so I press enter and I can hear the DVD whirring and stopping a few times, then it tells me the error message again.

I've set the CD/DVD drive to boot first.
I've checked the wiring.
The Vista DVD seems to work fine on this computer. I didn't try to install.
I'm going to try to boot from a USB device...

Also, the power switch wire on the Antec 900 doesn't seem to get along with the mobo, so I've been starting it with a screwdriver by shorting the two power on pins. I think I'll take the top off and check the wiring for the front panel. It seems like it's shorting out somewhere...

Any ideas?