Ulp, started responding one by one before I realized how many excellent and thoughtful replies had been posted.  Will try to consolidate my own responses to avoid having all these little individual tguyus responses in the thread.  And I really really appreciate ALL the time and thought folks put into these responses.  What a great community...

Quote - Check out that thread. I was asking for help putting together a new system last month. I got some seriously good advice and information and now have a system that I'm happy with.

Will check out the thread referenced by Acadia... thanks!

Quote - i would upgrade the graphic card to 512mb, up your hard drive gigs too.
make sure you have sufficent cooling for the beast.

Will look into 512mb gfx card... though I wonder if just plugging 2 256mb cards into an SLI arrangement wouldn't be plenty of power for running any of the games my son wants to run (e.g., Oblivion).  Will also explore HDD options more thoroughly too, though there I might just stick with a single fast drive for the system and core programs (or maybe 2 identical in RAID 0 or RAID 1) plus another big (maybe slower and cheaper) drive for storage.  So many options!  Thanks...

Quote - Spring for more memory, at least 4gb.  Since you're mainly going to use Poser there isn't any huge advantage in using XP 64 with only 2gb.  If you have 4gb you can at least use a whole 3gb of RAM for only poser, with 1gb free for the OS and whatnot.

If you have a choice between two produces of the same model, and one is "Retail" e.g. your hard disk, and the other is "OEM", always get the OEM version.  It is much, much cheaper.  Look for an OEM package hard disk and I'm certain you can get much more space for similar price.

Louguet here has shown that SLI (two video cards) doesn't benefit appreciably in Poser and other 3d apps.  I wouldn't even consider it.

Consider going with a simpler Soundblaster card.  I doubt anyone can really appreciate the difference between them.

All excellent thoughts, thank you.  I will definitely look into the cost of upping to 2 sticks of 2G RAM.  That would give me 4gb with capacity to expand to 8gb if that ever seems worthwhile.  I'll also pay attention to the retail vs OEM thing.  I want to make sure I get all the setup disks and cables and such that I need, so I guess I will need to be careful about what I can do OEM (HDDs?) and what really needs to be retail (motherboard?).  I did notice that on newegg.com the differences between OEM and retail version prices are pretty small for some things.  But I will definitely check!  And I will dump the sound card, esp since another kind poster suggested the onboard 7.1 should be fine.  Thanks!