Looking at an offer now, computer best described as for gaming --

although mainly I'll be using it for other things. I explain things a bit better here.

It's fairly urgent though. I'm keeping my screen (22" LCD) and power supplier (650W, nearly brand-new, can't seem to find any of the specs at the moment). Otherwise, this is what they're offering:

i5-750 LGA 1156 Package 2.66GHz processor
Intel Whitesburg DP55WB motherboard
Crusial (Micron) 4GB DDR-3 1333MHz (2x2GB) RAM (Reputable manufactorer? Do I realistically need more?)
Gainward BLISS Geoforce GTS-250, 512MB

Other parts are largely incidental. Also an option to upgrade to a Warrensburg DP55WG for approximately fifty Dollars, and a Gainward GTX-275, 896MB(possibly 1G? Not sure) for circa 200. The first I'll probably take. The graphics-card I'm rather unclear about, but I understand there will be new models coming-out soon, possibly effecting prices if nothing else. Similarly, I've been given to understand the i5 motherboards may have longer shelf-lives than the i7's? I could really use some advice in general, as well as specifically about the above components, or anything else I should keep in mind.

(My budget's fairly flexible at this point, but of-course I'd prefer to be cost-effective.)

The i5-750 is a good cpu. It will run the same on any P55 based motherboard. Buy the least expensive P55 motherboard that has the features that you need, and don't pay for an enthusiast motherboard unless you anticipate multiple vga cards or record breaking overclocking. Anything form Intel, Asus, EVGA, or Gigabyte should be reliable. Don't worry about obsolescense, by the time you replace the i5-750, the new cpu's will require a new chipset anyway.

As to the graphics card, it is the most important part for gaming. A 22" monitor will usually be 1680 x 1050. How good does your gaming need to be? Here is a good article on what it takes:
http://www.guru3d.com/category/vga_charts/

The graphics card market is quite competitive. You mostly get what you pay for. The GTS250(aka 9800GTX) would serve you well. A ati 4850 is comparable. A step up would be a GTX260-216 or GTX275, or the 4870 or 4890.

Yeah, I'm pretty sure I'll take the P55WB. If nothing else, it'd like to have the option to SLI, faint as it is, and it appears to be a good board all around. I think the GTS250 should be plenty for me at the moment, though I'll probably spring the extra ~20$ for a full giga RAM. Resolution really isn't a big concern for me by and large - I actually notch it down significantly under most circumstances to see better (my eyesight's rather wretched - it's the reason why I have a 22" screen in the first place, and why I prefer not to overclock or otherwise fiddle with my own hardware). Games are a bit different, of-course - for one thing, there's hardly any reading - but I do feel just a little self-conscious about paying for a card I'd constantly underwork.

I'll probably place an order tomorrow. The only last minute considerations left are the case and the option of going with an i7-860 instead. The price would be quite manageable, I'm inclined to take it at the moment, but I'm wondering how much of a boost it really represents, and whether I'd be better off just going with a 920 and shelling-out a bit more on a p58 at that point.

Incidentally, I've finally managed to track-down the specs for my PCU - it's a HEC Couger 650W.