You have three HDs listed in your setup. I'm not sure if there are too many SFF cases that offer three HD bays (unless you forgo the optical drive and use a 5 1/4 bay to hold a HD). You could get an external USB 2.0 case to hold one of the HDs if all three are essential. Also, when you talk about 400 mhz DDR support, I'm assuming you're talking solely about memory and NOT about FSB support? Or is your OC done through the FSB in this case and you need FSB support up to 400mhz?
This should be fine, although I don't know about how big your tuner card is and how well it would co-exist in that tight of a space with the 9800. I've never built an SFF AMD based PC so I can't speak intelligently about any issues that may be encountered. I've put together two P4 based ones (SS51G w/ 2Ghz P4 & Ti4200 & no-name Chyang Fun (?) with a 1.7 Cel and a Radeon 8500) and they have been solid and reliable (though at times a bit noisy). But neither of these systems are OC'd, nor are all the drive bays full (1 HD, 1 optical, no floppies) so I've never had power/power supply related issues that you may encounter with more/higher end peripherals drawing power. Also, I dont think this Biostar or the Shuttles officially support the 400 mhz FSB at the moment (I'm not sure that they're even capable of it, again never having built a SFF AMD box before).
EDIT:
OOPS, just noticed the Biostar only has 1 3.5" bay. The Shuttles have two but I guess these Biostars only have one. This has SATA though, which the two popular Shuttle variations do not (
SN41G2 and
SN45G). You could put an SATA card in the PCI slot of a Shuttle, but then you couldn't use your tuner card. These small systems are often about compromise.

Also, read the customer comments at Newegg, they mention some issues with the SATA and possibly problems with some high end video cards for this Biostar.
Check out this
Soltek too. Again, no SATA though it does explicitly support the 400mhz FSB. As does the Shuttle SN45G, I've now noticed.