1st homebuilt in nearly 3 years...update me plz!

hi gents & dames. long time lurker, first time poster. i've been out of the loop on tech for quite awhile as you can see by the title of this thread. i've been building and maintaining my own systems since my first pc in 1999. i like to game, but it's been mmorpg's for the last few years. it's time for an upgrade.

here's my problem...EVERYTHING has changed this go around, whereas typically i've only had to buy a new vid card and/or some memory. now we're into 64-bit and dual-core processors, SLI and crossfire tech, and SATA peripherals. thus, i'm a little lost.

i'm looking for a dumbed-down primer on the current state of things. i'm looking for a $400 or less build leaning toward graphical power. aside from the few games that i'm interested in (City of Heroes 4Lyfe!), i'm a utilitarian user with a splash of audio editing. here are some specific questions that you may be able to help me with (please be gentle, as i could be considered a newb with all of the tech that i've missed over the years, and some of these questions may not make sense on account of my ignorance):

-is there any danger that a 64-bit processor running XP home will not run older games (sometimes i install Grim Fandango for old-school kicks)?
-will a mobo with ATI on-board graphics have any trouble running an SLI-equipped nVidia card that is running alone?
-is an air-cooled CPU a thing of the past, or still standard procedure?
-is the assembly of a CPU cooling fan/heat sink combo still the same procedure as it was 3 years ago? (read: arctic silver)
-do i NEED a psu larger than 400W?
-do these things exist: old style power connector-to-serial power connector adapter (do i need a new PSU because of serial power?), IDE-to-SATA adapter (my DVD burner isn't more than 18 months old)

i didn't want to post a 'how do you like this combo of hardware' thread, since i'm flying nearly blind. there are plenty of these threads, anyhew. i thought one like this might be helpful to another old schooler like me.