What parts you need to have depends on what you want your HTPC to do. For example, if you plan to store many audio/video files in it, you need hard drive space. Thus, while shopping the websites of the four main PC makers, you'd lean toward the biggest hard drive options they offer, so you could start off with the biggest hard drive you can, probably 120-160 or maybe eevn 180 when you're dealing with the big four PC manufacturers. But if you'd rather play your CDs and DVDs in the optical drive off the original disks instead of from hard-drive backups, hard drive space won't matter, so you can save money with the basic level, which is typically 40 GB. (You'd also start at that level if you plan to put the OS and programs on one hard drive and media files on another, because the biggest hard drives in the 200-300 GB range are something you'd buy separately, not as part of a pre-assembled package.)
Note what kind of video card the pre-made PCs come with. ATI/Radeon cards are considered the best line around here for TV and movies, although I think maybe I've seen it said that N-Vidia cards can be taken seriously by "gamers". At least one company gives you the choice between the two.
You would need Powerstrip if your display device thinks like a TV; you don't if it's a true computer monitor and doesn't have some wacky native resolution. If you don't already have a display device, you're better off getting a monitor than a TV.
You'll probably want the monitor and speakers of your own separate choosing, so watch for no-monitor options and don't pay extra for the little doopy speakers many PCs come with.
Other programs, you'll need or not need according to whether you'd use them, obviously. If you plan to store music files on the computer, you'll need a program of the type called "jukeboxes". If you want to watch DVDs, you'll need a DVD-watching program. If you want to backup DVDs on your hard drive, you'll need a "ripper".
If you plan to watch TV, you need a tuner card. Those generally come with the software you'd need to use them. Just watch out for which ones are "standard" (480i) TV only and which ones are "digital" or "HD". Only get the card for the type of viewing you plan to do.
If you plan to feed input from any other devices like a Sega/Nintendo game system or VCR into the HTPC, you'll need a capture card with the right kind of inputs. Sometimes, you can get all the capture inputs you need on the same card with the tuner. There are lots of capture and tuner cards to choose from depending on what you want to plug in to them.
When the PC manufacturer's website asks you what kind of memory and CPU speed to get, there's no need to get fancy and flashy. Many HTPCers even make the case that you're better off lagging a bit behind the "bleeding edge" on those things so you can get technology with some maturity and refinement and stability. The "power" to do HTPC functions already exists in any CPU made new these days, except maybe the very cheapest IF you're after HDTV. And most HTPCers don't even really push 512 MB of memory usage.
