Its a bit late, but I thought I'd share my experience.
I did a similar build into an old dell dimension 2400.
SATA wasn't a problem and I have 2* startech 4 port SATA PCI cards working really well (plus a PCI Gbit network card). PCI 33 means cards are only SATA1 speed but thats plenty for streaming movies.
Operating system caused major issues though.
I started out using an existing XP home installation with shares and then tried moving to various linux installs, which were initially promising but my (and I think your?) 845 chipset with built in intel graphics doesn't work well with the linux kernel (i.e. all flavours of linux are affected). (google linux intel 8xx 9xx graphics etc for further info.)
Basically the graphics kept crashing and took the network card down as well, and as theres nothing like rebooting in the middle of a movie to double your fun, I went back to xp home and shares until they fix the issue.
I guess this MAY affect your choice of unraid (which i think is linux based?)
For "backup" (OK, lets call it resilience rather than backup) I didn't have anything at all for ages, but the fear got to me and I now have 2* 1Tb drives mirrored as RAID 1 on one sata card for data that changes a lot, and 4* 1.5Tb drives for media (3 with actual data and one with parity updated nightly using flexraid, which is free and fantastic).
All of these drives plus the original IDE system drive run happily off the original dell PSU.
I recommend Flexraid highly for the fact that it deals well with drives which:
a) contain data already and
b) don't change much (i.e. media),
c) live under any operating system
After the initial (in my case 14 hour) parity calculation it updates quickly (e.g. after adding about 25GB of DVD ISO data the parity update lasts about 20 minutes)
Hope this helps and have fun
Added bit:
WRT Geebas comments: Reliability could be an issue, but my experiences with installing and configuring ubuntu (et al.) on my existing box have taught me that if something fundamental happens to the machine I could get new board and case, plug the drives in and have everything looking identical (on the network) in a few hours.
Reliability isn't just about the age of the machine, its about how quickly you could replicate and replace it when it goes wrong (whatever the age
Fundamental to that (I think) is disks which are resilient in some way (raid1 and flexraid for me) and readable anywhere (i.e. not reliant on any particular hardware configuration or OS).