
Well the first deciding factor is Intel or AMD. Then it comes down to the chipset you go for and the brand/manu of the final mobo. Here's a bit more info:

Intel love to change their mobo archy leaving users stuck with out-dated mobos and over-priced upgrade routes, currently Skt478 is the latest and def what you should go for. Things you will want from an Intel P4 mobo (that includes the latest Celerons 1.7ghz+) are little more than the ability to run for as long as possible (Intel may be revising again at 3ghz so it is poss NO current mobo will do) and also the ability to run a 533FSB as well as 400FSB. Intel CPUs run very cool (and pretty quiet) and the 512k 400FSB ones are great o/c'ers so you can easily get a lot more perf out of them for free, but I doubt you'll be overly interested in that. P4 CPUs are very expensive, hence why the Celeron is still around and now based on a cut-down P4. Celerons are very slow, the Celeron 2.0ghz is the fastest and uses the 0.13mu meaning cooler, quieter running and it should o/c to 2.6ghz quite easily but will still be noticably slower than a P4 2.0ghz. So if you do want Intel go for a P4 not a Celeron. Skt478 has the options to gor with DDR or RIMM-RAMBUS, forget Rambus it is NOT worth the added cost. So quite simply be sure to get a Skt478 mobo that uses DDR333-PC2700 (the slowest you should use with the hungry P4) and has the ability to run at 533FSB as well as 400FSB. Chipset doesn't matter hugely beyond these factors, you should find things like USB2, ATA133, SerialATA150, AGP8x, RAID are common but don't worry about them as USB2 is the nicest one to have and even that isn't hugely useful yet. You can get onboard (o/b) stuff (built in to the mobo) with many mobos, gfx, sound, LAN etc but again these tend to be very basic solutions. The o/b gfx are poor but if you won't game then they will more than suffice, just be 100% sure that the mobo does have an AGP slot so you can put a good dedicated gfx card in at a later date should you wish to.

For AMD SktA is all you need although to have the best compatability witht he top CPUs there are certain chipsets to look out for. AMD do the Duron at 1.3ghz which is about as fast as a Celeron 1.7ghz, but AMD are phasing the Durons out and the AthlonXP start pretty cheap and are much faster. In terms of speed AthlonXP1800+ = P4 1.8ghz, AthlonXP2000+ = P4 2.0ghz etc. All but the very top AthlonXP are very cost effective. Again I doubt o/c'ing is important to you, but AthlonXP2000+ to XP2200+ are the ones to avoid if you do want to tweak them. RAM-wise you should find all current SktA mobos use DDR, DDR-PC2100 is certainly all you need although DDR333-PC2700 isn't much faster it costs only a little more and will allow for more tweaking and future-proofing. All but the AthlonXP2700+ and XP2800+ use a 266FSB which all remotely modern SktA mobos can handle, if you may want the top CPUs then you simply need to verify the mobo can handle the 333FSB these CPUs use. Chipset certainly matters for SktA and without getting too deep here they are: 1. VIA KT333/KT400 are cheap and very tweakable (hence o/c'able) but are the quirkiest and most problematic SktA chipsets. 2. SiS 735/745 are again cheap and lack tweakability but are rock solid stable. 3. nForce1 (215, 220, 415, 420) are much like SiS but have excellent o/b stuff, namely great 5.1 sound and great LAN and the 220 and 420 have GF2MX gfx which are slow but about the best o/b gfx options. If you want the o/b gfx choose 420 and use 2 sticks of RAM as this enables Dual Channel RAM to speed up the o/b gfx. nForce2 is VERY new but should bring it all, speed, tweakability, stability, future-proofing and great o/b stuff ... the gfx are upgraded to GF4MX which isn't hugely better but is a good step up from GF2MX, again Dual Channel RAM makes a big diff to the o/b gfx although it now gives everything a little boost too. Other than the nForce the o/b stuff on SktA is much the same as Intel's Skt478.

So do you want Intel or AMD? The mobo and RAM Mpretty much evens out so the price diff comes down almost completely to the CPU alone:
AthlonXP1800+ $74
AthlonXP2000+ $93
Celeron 2.0ghz $113
P4 1.8ghz 400FSB $120
P4 2.0ghz 400FSB $144
P4 2.4ghz 533FSB $188 (533FSB gives P4 a good boost)

I'd suggest AthlonXP2000+ on an nForce1 or nForce2 mobo, nForce1 is mighty fine but won't support 333FSB that the top CPUs use, the nForce2 has slightly better o/b stuff and is very tweakable, fast and more future-proof. Either way you'll get adequate o/b gfx, great o/b sound and LAN. You really can't beat that for 'bang for buck'. If you want to game a little more then go SiS 735 or 745 and add a great and cost-effective AGP gfx card like the Rad8500LE-128MB, GF3 or GF4TI4200 all of which are in the region of $100.