I'll go ahead and ask, why hasn't the OP named his most demanding apps? Tell us what you're using and we can tell you what you really need. Saying you're using basic programs and maybe whatever games it can handle doesn't really say anything. If games were your priority, I'd balance the build to be a little GPU heavy with a CPU that isn't too much of a bottle neck. If you're into video editing/encoding, I'd balance it to be CPU heavy. If you only use a browser and Outlook, I'd tell you to get the cheapest thing that wont fall apart on you in the next 5 years.
Without this knowledge, I can only give you more generalized advice. Don't neglect your PSU, as said before for the obvious reasons that a poor PSU will likely bring you to PC ruin(I recommend a 80+ certified from a reputable company, modular are a bonus). Get only as much RAM as you actually need. XP only really needs 2GB but Vista or 7 64bit would benefit from 4GB. Dual Core CPUs are absolutely fine as long as you don't run CPU intensive software like video editing, then you might want to bump up to a quad.
Spending more to get a i7 build when you aren't even into gaming/video editing/etc. to "future proof" is nonsense. I can't tell you how many times I jumped on the bandwagon of a new tech only to see it replaced or "upgraded" within a single year. It'll be at least a year or two before they start to really slow down production of LGA 775 CPUs. Not that it should matter to a non-gamer. Chances are, if you have to replace your CPU, you're replacing your motherboard too anyways. You can pretty much count on it. Every single time I tried to buy a motherboard with the intent of upgrading my CPU in a year or two, I had to buy a new motherboard to upgrade the CPU, for some reason or another. Even if i7 builds are still the thing when you go to upgrade your CPU, you'll probably find your motherboard doesn't support the bus speeds of the new CPUs or some other compatibility issue.
Cases are pretty much a personal preference. Find the case that holds what you want and looks like what you wouldn't mind working in and just generally suits your tastes(if you build your own).
Only go with AMD, if you're doing a super cheap budget build but your $650 budget can accommodate an Intel build. AMD is good for a budget Linux build and that's about it these days. Maybe there's some niche I missed but that's generalized advice for you. Also, it's possible AMD caught up to Intel somehow while I wasn't looking but I doubt it.
The thing about hard drives is you have to choose between cheap storage(those 1TB or higher drives), fast loading for apps(Raptor) or super fast loading and no moving parts(SSD). The Raptors are faster but they aren't so much faster that you can't live without one. Only get one if your budget has room for it. If you were building a dream machine(no real budget limits) then I'd say SSDs are superb because, if you raid those suckers, you can load stuff almost instantly(an entire DVD can be copied in only a few seconds). There's a video on youtube where some guy put like 40 high performance SSDs in raid and he was reading/writing so fast it was insane. The only thing about SSDs is the inherent problem of questionable longevity. They're made of ram and ram is volatile by nature. I don't feel like getting into detail about wear leveling or other such stuff since SSDs are out of your price range anyways(they're currently out of my price range too). I just know I drool at the thought of a raiding SSDs to put my windows install and all of my games on. There would be no loading time at all on a setup like that, even on the biggest maps.
