ciphergoth | Building my own media centre PC

Now we have a jolly huge TV and a GBP 25 DVD player, I find myself very tempted to build myself a Linux-based Media Centre PC. I'd love some advice on what I'm thinking of buying.

I want:

Linux-based media centre PC running MythTV.
Fits on TV stand (thus desktop case)
Fast enough for software decoding while doing other stuff
Not too noisy
Ideally one that doesn't use too much power (environmental impact)
Cheap
~~Slight preference for AMD 64 for coolness value.~~ (given up on this one)

I don't want

Games machine (will never run Windows so no point)
Very quiet indeed (no-one in the house seems to mind PC roar)
Lots of compatibility hassles making it work

I already have

DVD +-RW rewriter (ATA)
AGP graphics card with TV out (NVidia MX400 MS-8826)
Cheap sound card if I can't make the onboard one work with Linux

I'm thinking of buying: (prices include VAT) Updated with cheaper, quieter, slower choices.

56.09	Processor and fan	2.66GHz Intel Celeron 330D, 256K L2, socket 478, 533MHz FSB
97.98	Motherboard, case, PSU	Black ASUS Pundit barebones system
45.88	Memory	Ebuyer 512mb DDR PC3200 400MHz Tesco Value Ram
76.73	Hard drive	200 Gb Seagate Barracuda
62.64	TV card	Hauppage DVB-T PCI Nova 909
6.68	Video cable	Belkin PC-DVD to TV Cable Kit
5.86	Postage and packing
352.76	Total cost

So, what have I forgotten? What am I buying wrong? What won't work with what? What do I need to check about my existing bits of hardware? Where should I be saving money/spending more?