personally, i use a pentium m 2.0ghz on my secondary desktop, and a p4-2.8 on my primary. my previous computer was a dual athlon 1.4, on the og k7thunder.
anyways, you could have justposted this on the tech board. the only person who matters that is responding is me
1) yes, the higher end p4's are gonna be bigger power sucks, because this number is usually pretty strongly correlated with the clock speed of the chip. intel chips run at higher clock speeds, thus higher power/temperature (not always the case, as the # of gates matter too, but the clock speed difference is better in the comparison between amd and intel).
2) 9800xt is an ATI video card? soyo dragon board is an athlon wonderboard? is that what you're running now?
3) nothing _needs_ a great power supply, but a good one certainly cuts back your rate of failure on your computer. I would have to say power supply failure is one of the more common failures in PCs now. And since a lot of these newer chips and components are increasing their power usage, it does force the power supply to output more power than before. a company like abit cannot put to market a motherboard that requires some high end power supply or high quality power supply if they care about their bottom line, so it would still probably work ok with whatever you got (as long as it's at least around 250W).
4) you most likely don't need a new case, unless you're buying a specialty power supply (which i highly doubt, since they're expensive and role based). with that said, new cases are pretty darn cheap, and can make your computer look cooler, and feel like a new computer, despite the fact that it might not be entirely new.
5) how powerful a power supply do you need?
If this computer is just for standard game playing and doesn't have too much shit in there (just like 1-2 hard drives, sound card, video, ethernet, 1 dvd burner, 1 cpu/mobo), a standard 300w will do you fine these days. brand? there's no definitive bests, but lots of people have preferences. with that said, just pick a name brand one and you should be fine. Deer, E-power, Enermax, NMB, PC Power/Cooling, etc..
if this computer has a ton of shit, just start doing some basic math. the high end intels probably suck up about 120W at full load. AMDs probably around 100-110W. (just numbers off the top of my head.. may be off.. don't hold me liable). Your video card runs about 30-90w depending on how powerful that shit is (if it's a 9800xt, probably around 50w). then tack on an additional 2-3 watts per fan, and like 20-30 watts per peripheral, and then get a power supply slightly bigger than that #. 300 is probably fine in most cases, some people like to be extra safe and get a 350w one. some people will completely bullshit you b/c they've been reading too much anandtech/toms hardware bullshit to tell you you need a high quality 400w+ one. bullshit.
6) i'm gonna go out on a limb here and recommend you getting a pentium M. little more pricey, but they are sweet

I think abit or some company makes a pentium m (479->478 converter) for use on 875p motherboards (which are dirt cheap). they're just as fast as their desktop counterparts, run at considerably lower power levels, and because of that, you can put silent or even fanless heatsinks to cool it. that way your computer doesn't sound like fucking LAX when you're playing unreal. i'm using this paired with a lianli v1200 case + epower lion 450w silent powersupply. i also have 8 hard drives in this machine (6 in raid 5, 2 in raid 1), and the computer is extremely quiet still
