Vista 32 or 64bit

This is my current configuration:

(Alienware Area 51-7500)

ASUS P5ND2-SLI NFORCE4 MB
P4 3.0GHZ 830 2MB L2 CACHE Dual Core
2 x CRUCIAL 512MB DDR2 PC5300
16X DVD-ROM REV.2
DESKSTAR 160GB 8MB CACHE SERIAL ATA HD
LITE-ON 52/32/52 CD-RW BLACK REV.1
1.44MB FLOPPY DRIVE BLACK
CREATIVE SOUND BLASTER AUDIGY 2 ZS 7.1
EVGA NVIDIA GEFORCE 7800 GTX 256MB PCIE
MICROSOFT WINDOWS XP
650 WATT ATX2.0 POWER SUPPLY WITH ACTIVE PFC

I plan on upgrading my system this summer to Vista Premium, I know, I know, but I figure why avoid the inevitable. But I have some concerns, as I am worried about compaitablity with my parts. [I realize that for Vista Premium I will have to get a DVD-RW].

Three things really concern me. One is maximum memory, I am thinking about jettisoning the combined gig of ram that I have for four gigs of ram. But to do so, and really get the benefits, I believe I have to run Vista 64 bit as opposed to Vista 32.

Concern #2:

My wife is a budding artist and is drooling all over Photoshop CS3, which is Vista compaitable with 64 bit (at least I remember reading that on adobe's site   ), but would it be better to run in on the 32 bit platform.

Concern #3:

Until I make the leap, I want to make sure that I am not wasting money on memory that I cannot capitalize on. For example, I have read that Win Xp can only handle a maximum of 4gbs of ram, but really shows less than that because various other items, including pci-express cards, take up some of that availability. There also seems to be some sort of conflict issue with Asus boards and maximum ram allowed. Asus link to their faq

Concern #3A: Anti-virus software. Are there any designed to work with Vista 64 bit?

I am asking for help, on how to make a smart educated decision on which operating system to choose.

Any and all help would be appreciated.