Are you committed to a particular motherboard already, or AMD versus Intel? One idea is the MSI "Hera" line, here is a page which lists the models (it's near the bottom of the page):
http://www.msicomputer.com/product/all_products/Productindex.htm
In a "slim" case, I would lean toward a Tualatin-based Celeron such as the 1.0A in order to get a mix of low heat production with medium performance level, and I found the MSI MS-6215T at TechStore:
http://www.techstore.com/solutions/item.asp?sku=621023TD Judging by MSI's page and photos, this comes with built-in Firewire, front USB and audio ports, TV-out, and *might* come with a DVD-ROM drive and floppy, which would explain the $200+ price. However, it is an LPX-style case, not microATX, and would not take a microATX board. LPX or NLX cases allow for very slim cases that still accept normal PCI cards since the cards end up parallel to the motherboard.
Do a search for "bbns" at TechStore and you'll also see the $125
MSI Hera, an AMD-based model, but I've never seen them report it as being in-stock, and with the power demands of AMD processors, you probably would not want to put in anything more than a Duron 800 or so. This one takes *low-profile* PCI cards (rare) and also has a riser card for one normal-profile PCI card, so I don't know what the heck that motherboard must look like.
😉 But it's a safe bet it's not "normal" microATX stuff.
For a standard microATX desktop case you might as well look at the usual suspects like Enlight and Inwin, but they're not going to wow you with their compact size. However, you won't be restricted by weird motherboard formats and riser cards either. Frankly, having owned a BookPC once, I'm done with that proprietary nonsense once and for all.
Anyone else have any other suggestions?