i'll try to break down your questions and answer them one at a time...
as far as websites...i only really know pricewatch, don't be affraid of it. you will likely find the best deals there.
as far as processor/mobo, there are a bunch of things to consider. when i first bought a t-bird, i thought i wouldn't want to try to overclock. within a week, i was mad i bought a board that didn't offer it...so don't limit yourself. although prices are a bit higher, i would suggest buying a board with the kt133A chipset (instead of one with the regular kt133) for future upgrades (so it can handle the 266mhz fsb t-birds). the general consensus is that abit and asus are the top (obviously opinions vary). i personally have no complaints about my epox mobo so to save some money you might want to look to epox or msi which both make good boards. now for the cpu...it seems that the 600/650 durons are the best overclockers but out of the box the 700 seems to be the best bang for the buck. the t-birds do have a performance advantage of roughly 100mhz (maybe 150mhz, i.e. a 700mhz t-bird = 800-850mhz duron, but only with programs that need the extra cache, some programs, such as those that only need raw fpu power will see no gain since the chips are virtually the same aside from the cache). that being said, it's up to you. if you want a t-bird, i would either get a 750mhz or a 900mhz chip depending on how much you want to spend. those speeds seem like the best bang for the buck for their price.
now for ram...definately get pc133 with current prices so low. the thing with the kt133 (and kt133a) chipsets is that you can run the ram asynchronously. the regular t-bird (and duron) has a 100mhz fsb (ddr makes it an effective 200mhz). it therefore uses a 1/3 pci divider so that the pci slots run that the correct 33mhz. the way it uses pc133 ram is it adds the pci clock to the host clock (fsb) without changing the pci clock (meaning the pci slots run at 100mhz/3=33mhz and the ram could run at, if you choose, the pci clock of 100mhz/3 + the host clock of 100mhz = 133mhz. it's not as complicated as it might sound, i'm just not the greatest teacher. the point is that by using pc133 ram you are not "overclocking" the sound or video cards.
an overall rough estimate (on the high side, with shipping) with good parts would be as follows:
mobo = $150 (for abit/asus)
ram (256 megs pc133 cas2) = $100 (crucial)
cpu + good heatsink/fan = $90-$150 depending on speed/type choice
this would put you in your $350-$400 price range. if you shop around and go for different name brands i'm sure you can find this upgrade will cost closer to $300.
i hope this ranting and raving helps.....