For mobos it tends to be the chipset which is more important than the manu, although if you have found a partic manu to be good there's no reason not to stick with them providing the price diffs aren't huge. So on to SktA mobo chipsets:
VIA: KT333 isn't hugely better than KT266A, basicly can run PC2700 async (3% perf boost) and has the PCI divider for 333FSB although it isn't guaranteed to run at 333FSB. KT400 is the newest and should be the most future proof. It unoff allows PC3200 and 400FSB while 333FSB is now certified and things like ATA133, USB2 and AGP8x are now built in as standard. IIRC KT400 overcomes the quirky VIA PCI bus which has often proved problematic with PCI RAID and SB Live soundcards. In a nutshell VIA tend to be the fastest (only slightly) and certainly VERY tweakable and o/c'able (nForce2 is also very good on both of these counts) but VIA although cheap and popular are known to be the quirkiest mobo chipsets going.
SiS: 735 and 745 are the most available and there's very little diff between them, IIRC 745 simply allows async PC2700 but it is slower than synced PC2100 so don't buy it for that! These are known to be very robust, compatible and stable chipsets but severely lacking in the tweaking and o/c'ing dept.
nVidia nForce: nForce (nForce1) come in a variety of flavours and are very similar to SiS (very comp and stable but lack tweaking) but with great o/b goodies. Types are 215, 220, 415, 420 and some have the 'D' added to the end which means they do Dolby 5.1 sound. The 4__ have Dual Channel RAM (reqs 2 sticks) but it's pointless unless you use the o/b gfx which are GF2MX200 and built in to the _20, very underpowered for even basic gaming. The things they have in common is great o/b sound and great o/b LAN, they do all tend to be quite pricey though. nForce2 are just out and upgrade a number of things most notably the GF4MX420 o/b gfx which is much better than GF2MX but still very underpowered, dual LAN, better Dual Channel RAM, ATA133, AGP8x, USB2, firewire and 333FSB support along with PC2700 and unoff PC3200 support. Along with VIA KT400 the nForce2 is by far the most future proof chipset although KT333 should be pretty fine too.
AMD: Haven't released anything new in ages and are very outdated now. IMHO the next AMD chipset will be for Hammer.
So if you want speed and tweakability go VIA, if you want good speed and want stability get SiS or if you want great o/b too nForce1. If you want the best long term solution VIA-KT400 or nForce2 are the clear winners. KT400 is meant to be fundamentily diff in many ways to the older VIA chipsets and a new PCI bus which may enable great stability, although if you want it all (speed, stability, features, tweakability, lastability etc) it seems nForce2 is def the way to go. As for manu, ask around, but generally just because brandA made a great KT266A mobo does NOT mean their new KT400, SiS745 etc is any good, so only support is likely to remain the same but even that isn't true for co's like MSI. I'd say choose chipset, then look at prices to make your decision. Of course reusing your CPU is easy but if you still want SDR then that limits your choice severely, usually to either SiS735/745 or KT266 (note not the KT266A), you can get older SDR-only mobos but the ones mentioned often allow SDR now but also allow DDR as a future upgrade. Considering the price of DDR and AthlonXP1600+ to XP2000+ it makes a lot of sense to get more than just the mobo.