"How big of a HDD can this computer take and does it take SATA Drives"
According to the service manual the hard drive is a "1 inch high" SATA drive, I assume that means it's a 3.5 inch drive. a quick search didn't reveal anything on size limitations and I've never seen any mentioned for any other SATA systems so I strongly suspect means you won't run into any with HDDs currently on the market.

"What interface does the Slimline CD Drive use?"
Apparently it's on IDE, the good news is it's connected with a cable so with a suitable adaptor cable (search for "Mini SATA Cable for SATA Slimline Optical Drives" http://www.mini-itx.com/store/?c=5 ) and a low profile stata controller in the PCI slot or an IDE hard drive (unfortunately the motherboard only has one SATA port) you should be able to use a SATA optical drive

"How well does this computer run the Mythbuntu Operating system?"
No idea.

"what type of RAM does this take and what is the maximum size?"
The memory type is "400 & 533 MHz DDR2 SDRAM " the max memory (according to dell, in my experiance real maximums are sometimes higher than the manufacturers claim) is 2GB (2x1GB)

One big issue I see you having is expansion slots, there is only one PCI slot and one PCIe slot, both are low profile and based on experience with other dells it wouldn't surprise me if the PCIe slot is graphics only. This may be a problem if you are tyring to add both a SATA controller for the optical drive and a TV tuner. Maybe one of the ATI all-in wonder cards (combined graphics card and TV cards) is the soloution to that problem but I dunno what thier linux support is like and whether they are available in low profile.