I'm half tempted just to fork out for a new 64 bit system and at least get it the power without the hassle. There's some high end PCs reviewed in PC Pro this month - I'll post up some of their specs for your consideration later.
I can recommend it for graphics work just gone to 64 bit vista from 32 bit xp
so the skinny is...
most of it works fine the extra ram is wonderful (photoshop however has a theoretical limit of 3 gig even in 64 bit version).
the components i used were seagate baracudas (3.5 TB of the them)
6 Ram DDR2 800 (DDR 3 compatible board)
Quadcore c2d 6600
geforce 8800 gts dual monitor card (which again i'd have thought you'd want)
and a 750 w PSU
this little lot set me back around £600 in total tho was self build.
assuming that you can attach satas and dvd drives to motherboards then there's really minimal 'installation' to a pc these days and components have finally got simple sensible coupllings generally.
and you'll get a kick arse machine for less...
in terms of VIsta 64 install it took no longer than a standard vista install, worked out the box and with the exception of codecs for raw files (which is a touch shit) then you'll have no problems.
From build to first boot to first use as a computer was around 6 hours (2 of which was using the jolly simple transfer my setting function in vista which pulls through near enough all your old settings programs and ting from xp download the
vista compatiblity tool which can tell you if you are going to lose out by going to 64.
whilst spending say 6 hours fiddling in the innards of a machine and setting it up might not be your idea of fun, you have to spend that time sorting out a new machine with your programs on it before you can use it if you buy off the shelf.
I'd also recommend buying caddies for any storage in the old pc which you want the data off this can reduce the amount of time needed to build as you simply treat the old disks as external hard drives.
ddr2 800 is fantastically cheap at the moment for 4 gigs (around £70)
I'd recommend any asus p5 series board really (haven't had any issues with machine's, (around 12) I've built based around this board, all been bullet proof thus far)
for graphics work then i'd also chose a quad over a standard c2d
the only potential i can see in this being slightly hard work for you will be fitting the cpu cooler due to the simplfied (yet slightly annoying) system for attaching it which doesn't with the kit cooler with the chip at least feel terribly secure...