Never go cheap on the mainboard. Just don't do it, buy quality mainboards and you'll have far fewer problems. As long as you stay away from the fancy newer boards with things like DDR, you can do a cheap upgrade.
If you are thinking of going with a new CPU and mainboard plus RAM with a reasonable price, I would get:
Abit KT7E mainboard:
http://www.hardocp.com/reviews/mainboards/abit/kt7e/ Azzo has them for $115.00.
http://www.azzo.com/test2/mother.html Your choice of AMD CPU budget depending. If you can afford a new AXIA or later TBird, get one. If not, pick up a cheap Duron and OC the crap out of it. Lets check Azzo again just for a price.
http://www.azzo.com/test2/cpu.html#AMD Cheap route is a Duron 800 for $58.00. Overclock the snot out of it.
Better power is a T-bird: 1.2 Ghz Socket A is $129.
Either way you'll need a good Heatsink/fan.
http://www.azzo.com/test2/cooling.html Lets go high end for OC potential: Black Thermoengine with big hitter fan. $37.00
Complete the package with a 256 Meg stick of PC133 CAS2 SDRAM. It's SO CHEAP. Buy a namebrand like Crucial to get decent quality and still a good deal.
http://www.crucial.com Sell your 128 Meg stick of slower RAM... it's not really worth putting it in your upgraded box. If you can afford it, get 2 of the 256 PC133 sticks it's so cheap.
http://www.crucial.com/store/partspecs.asp?imodule=CT32M64S4D7E Free shipping... that's $125 for 512 Megs of good quality fast RAM.
Get a decent 300 Watt "AMD Aproved" PSU to go with it. Gotta figure you won't spend too much here. Lets say $30.
Upgrade to Kt7E, 512 megs of PC133 RAM, T-bird 1.2 Gig, good Heatsink/fan, new PSU. $ 437.00
Upgrade to Kt7E, 256 megs of PC133 RAM, Duron 800, good Heatsink/fan, new PSU. $ 302.00
Both would be nice rigs. The Duron box could be upgraded to a much higher spec Athlon and more RAM later if needed.