I wish I found this before I built my machine, because I had the same problems as you did and same goal. I wanted eyefinity + gaming + htpc. I finally did it, but it took so long to dig up informations and buy those parts. My machine:
Biostar G41-M7 MATX mb
2gb G.Skills Ram x 1
160gb 2.5mm HDDATI 5570 sapphire (same one)
Case - same
PSU - same but with a shuttle adapter
(I'm not sure how much watts I get from that but it seem to be powering it fine. 180w PSU)
I get good performance over the 3-screens in eyefinity setup. But, its just mmo and emulators.
The goal is to push it and see what kind of gig I can get under $400.
So the 5670 doesn't cover your ram clips? http://www.newegg.com/Product/Product.aspx?Item=N82E16813157236&cm_re=asrock_1155-_-13-157-236-_-Product
What about this motherboard? Will that heatsink on southbridge block it?
I'm also amazed that the 150w can power the i7 + 5670.
Do you think it will power a MATX + i3 + 5670 ?
I'm getting another case that's 9 x 11 x 5 so fitting them won't be a problem.
i actually upgraded to a 5670, unless you were referring to the old 5570 i used for like 10 minutes. everything is very tight, but the card does not touch the memory clips, nor the CPU cooler.
if this setup can power my i7 and 5670, i don't see why it can't power a i3 and 5670. the power difference between mATX and mini-ITX is pretty negligible, there's bigger differences between brands and platforms. mATX should be fine.
i actually have that asrock mATX. it's pretty much a hybrid between mini-ITX and mATX form factor. the width is just a tad larger than mini-ITX, but a lot smaller than most mATX. it takes up 3 expansion slots instead of 4, so you can fit it in some mini-ITX case. here's a quiet build i did for it so refer to this thread below. i just haven't posted it on this forum yet:http://hardforum.com/showthread.php?t=1612045