What chipset does the ECS K7AMA use? If it's KT266A, KT333, KT400, nForce, SiS735 or SiS745 I wouldn't rec you bother upgrading it, any of those should take at least any 266FSB AthlonXP (up to XP2400+?). Things that the newer mobos have since KT266A / SiS735 really aren't overly necessary at all. ATA133 isn't truly any better than ATA100 or even ATA66 FTM. Same goes for AGP8x over AGP4x, VERY minute diff and not worth spending money on at all, I doubt even the Rad9700PRO takes much hit (1%ish) by running in 4x mode. 333FSB only gets worth it for the XP2400+ and faster CPUs but even then 266FSB doesn't overly hurt perf, prob not worth spending the extra on a new mobo if that's all your current one won't do. Then there's USB2, about the only thing with a noticable perf gain but even then there's still very little which uses it and this can easily be added via a cheap PCI card if necessary anyway. Another matter is Barton support (AthlonXP with double the cache), I'm pretty sure only KT400 and nForce2 will accept it but I doubt anybody knows for sure, but a Barton with 512k L2 cache is unlikely to be much faster than current XP's and will certainly be expensive, SktA is reaching the end of the line (hello Hammer) which is another reason to avoid laying down too much cash if your current mobo is okay. So basicly it's unlikely that a mobo upgrade would be worth it. Oh, I nearly forgot you said you currently use SDR PC133, the perf hit from using SDR-PC133 over DDR-PC2100 is about 10% over all system perf but DDR is twice the price of SDR. I'd take 512MB SDR-PC133 over 256MB DDR-PC2100 but anyway if you upgrade your mobo it will mean roughly $50-60 new mobo, $60-110 for DDR (256MB-512MB) and you'll gain roughly 15% perf, a little better upgradability and a few little features like AGP8x, ATA133 and USB2. I'd rec spending your cash on a better gfx card first even though TI200 is still very decent!
So what would I do? If you absolutely need to upgrade then a gfx card would easily be tops. Rad9700 and 9700PRO are tops but they will devalue quickly and very few features will be of use for the price you pay. At $100ish a GF4TI4200 is a good choice but not hugely faster than your TI200 once it's o/c'ed, above this there's the 4400 and 4600 which aren't hugely faster but are priced about the same as the HUGELY superior Rad9500PRO which is what I'd go for personally, about $200. That leaves $150 to spend but XP2400+ and up are the only CPU upgrade worth it and they're still way overpriced (much like the P4 options). I don't think a new mobo will likely be worth it but you could get any mobo based on the KT400 ($70ish) or nForce2 ($100+) and of course DDR to go with it as we know 256MB $60 or 512MB $110.
I'd suggest you wait a few months as your system is more than capable and all of your upgrade options will be significantly decreasing over the next 8 weeks or so. If you want a boost now Rad9500PRO is IMHO the best option, see how you like it and then consider where to put the extra $150 (and consider keeping it in your pocket for a bit). If you wanted to a new mobo is more than fine and you could get 256MB now and then add more a little later. Lots of options but if you're itching for a speed boost Rad9500PRO is the only sensible and 'bang for buck' option (again IMHO).
ADDED: Oh do check the 9700 prices and 4400/4600 FTM as the prices should be changing all the time, if a 4400/4600 are under $150 then they are viable options and if a 9700 is close to $200 snap that up as the 256bit RAM bandwidth really gives a VERY nice boost. Another point is ATI cards and drivers, I'd trust them they are now VERY good indeed, simply changing the gfx card can often mean a fresh OS is needed and I think that's partly down to M$, ATI/nVidia trying to keep users loyal to them and also highly optimised drivers. Oh, do try o/c'ng your TI200 a little as you should get near (or exceed) GF3TI500 speeds and that may be all the boost you need for the moment and it's a free boost too!