Htpc + decent work machine - is it possible?

Hi,
I'm trying to put together a machine that could possibly double as a htpc (i.e. very silent) and a decent working machine. My budget is sort of limited, even though I can be convinced to break it if a significant upgrade can be made for only a small amount of money. I dont OC and I do not expect to be doing much gaming (at least not hardcore 3D stuff - mostly strategy). Here's what Ive got so far:

core 2 duo 6300 (183 $)
zalmanCNPS9500 cpu cooler ( 49$)
seagate baracuda 7200.10 SATA2 320 GB (read that these are silent 95$)
some DL dvd burner (~30 $)
hauppage 350 (150 $ need european PAL-version)

total expense so far: 500$

My budget would be around 650-700 $.

I need help choosing a stable motherboard that contains the following features:

SATA2 raid (could come in handy),
2-3 PCI (1 for the tvtuner, later option for a digital OTA tuner , WIFI)
1 ePCI (for a videocard see further down)
sound with digital out (coax or otherwise)
passively cooled northbridge

and I need advice on memory as well, how much is it worth paying? And which brand is reliable? 1gb should do it.

I would like to have a decent graphics card and it should be nvidea chipset as these are better suported under linux (I intend to run mythTV). It should have s-video-out (maybe even hdtv-out) and should be passively cooled.

I already have a powersupply, so I dont need a new one.

Im aware that this is overkill for a htpc, but I potentially need to do some work (numerical simulations) too so I want perfomance without getting too much heat and want a clear upgrade path.

Any comments and thoughts are appreciated.

just curious whats a htpc i been seeing that saying all over the place recently.