1) Go core 2 duo. Definitely. It's the best power-per-dollar ratio on the market right now, and blazing fast.

2) The T line of core 2 duo chips is designed for laptops. If you're building a desktop, get an E chip. I recommend either the E6300 (1.8 GHz, 1033 FSB, 2 MB shared L2 cache, about $180 USD) or, if you want to spend a bit more, the E6600 (2.5 GHz, 1033 FSB, 4 MB shared L2 cache, about $300 USD). I'm pulling those numbers off the top of my head, so they might be off a bit. The E6600 is the one I'd get, but if you want to save a bit, the E6300 is also a good bet. But really, if you're going to be spending money to put together a nice computer, you might as well go the extra bit to get the nicer chip. Up to you; both are fantastic power for the price.

3) Graphics cards. I would say, definitely get one, but don't spend a lot of money. With Vista out now, DX10 cards will soon flood the market and come down in price, but it's still insanity. You can get a $140 card ( http://www.newegg.com/Product/Produc...82E16814102068 ) that will play everything currently out as smooth as anything, plus it'll last you a while. I was surprised by this, my last graphics card cost me $400 (Sapphire Radeon 9800 Pro, 3 years ago) and now $100 cards can leave it in the dust.

4) "Caches". By far the most important and most overlooked thing by newbie computer builders. I'm not kidding when I say the extra 2 megs of L2 CPU cache are easily worth the extra $120, let alone the fact that the chip itself is faster. Cache is one area where more is *always* better. Faster memory, as well. It amazes me how many people build a computer with a scorching-fast extremely expensive CPU and low cache, slow motherboard, slow RAM, and a slow hard drive...

5) 64-bit. The chips I recommended are 64-bit capable. Don't worry about it. If you plan on getting Vista, then make sure you get the 64-bit version so you can enjoy the benefits, but for everything else, you pretty much don't need to worry about it. If you're seriously going to be putting this machine through its paces, make sure to get a compiler for x86-64, and make sure as much of your software as possible was compiled for it; software compiled for x86 will still work, but it won't take as much advantage of the 64 bit chip as possible. It's not a huge deal right now, since most things are still 32 bit only. Also if you put 64-bit vista on a machine, I hear everything stops working. Figures.

Finally, if you really are looking for a laptop (as your mentioning the T2250 suggests) then all my suggestions here are worthless. I would personally recommend a low-end mac, because their $1300 macbook is the best price/power ratio on the market right now, plus macs are just better (yay threadjacking into mac vs. ms arguments). Don't pay more than that, though, that's where the infamous "Mac Tax" *actually* kicks in. For high-end, the Dell XPS is my current favorite on the market now, and if you don't like macs, the Dell Latitude is the best for low-end PCs.