The HDMI cable should be good for 30'. They're supposed to maintain integrity for at least 50'. Go to Monoprice.com, its the cheapest price you'll find on HDMI cables anywhere.
You sure you want to build your own system? Its rare to have a system build go 100% smooth so if you aren't up for some potential need to tinker to get everything running you might want to go the OEM route, with Black Friday round the corner there will be some great deals.
If you still want to tackle a build here are the things I would suggest:
1. DO NOT cheap out on the power supply. You'll definitely need a new one, make it a good one. Go brand name and go high wattage. Get a nice one and everything will run more stable and you'll probably get two builds out of it, not one like most parts.
2. For your uses ram is king. Not just system memory, but also memory cache on your chip sets. Unfortunately cache size plays a big role in chip pricing in today's market. Here are three different 45nm quad core intel chips:
Intel Core 2 Quad Q8400 Yorkfield 2.66GHz 4MB L2 Cache $169
Intel Core i5-750 Lynnfield 2.66GHz 8MB L3 Cache LGA $199
Intel Core i7-920 Bloomfield 2.66GHz 4 x 256KB L2 Cache 8MB L3 Cache $289
So why three CPUs with the same clock speeds at different prices? Front side bus and memory cash. The bus is basically the speed data can be moved in and out of the caches, the caches feed data to your processors faster than any other connection in your system (unless you have a high end graphics card with its own caches). For memory intensive programs (multiple windows or having photoshop running with pretty much anything else) the key to having that fast responsive machine is memory.
For you I'd go absolutely no less than 4 GB of system memory, preferably more, which means 64 bit OS. By proxy, that means you want to go with Windows 7 Professional, Enterprise, or Ultimate. Those are the three flavors that offer Windows Virtual PC, that will let you run a virtual Windows XP without having to mess with a dual boot setup.
3. Your two 250GB SATA drives should be set up on a RAID if you want maximum speed. RAID 0 (striped) which means the two will work and be recognized by your system as a single drive. This is best implemented through a motherboard that supports a RAID 0 setup, many quality ones now do. If you want real speed I would even go so far as to suggest finding a motherboard that lets you have a RAID 0 setup as a slave drive (Intel's Matrix RAID is probably the best solution for that), then buy one of these:
Solid State Drives
You could probably get by with something in the 64-128GB range. Turn that into your primary boot drive, install the OS and your most heavily used applications on there, nothing else, then partition the RAID 0 slave drive into multiple content specific 'storage bins' like photos, movies, music, etc..
4. Even though you aren't an avid gamer I would still suggest a solid graphics card, it is very useful for things like photoshop and near imperative if you want to stream HD content while multitasking, as many players are beginning to incorporate some workload dump off to the GPU. Just get a solid mid-line gaming card, something in the $150 to $200 range, and you'll be set for a while. Shoot for a motherboard that allows SLI or Crossfire as well, that way if you want better graphics power down the road your old card isn't scraped, you just pair it with another.
As for your case, most anything will do but if you're building your own aim for something with clip in housing for drives, easily removable cages and side panels, etc.. Even then you'll want a box of band-aids handy before starting, but if you get a janky little mini-ATX you'll be bleeding before you know it.
I'd also strongly recommend going with a blu-ray drive that can record, preferably paired with a high read speed DVD/CD combo drive (doesn't need to burn, its just for reading when you spin off copies or other drive needs while burning a blu).