well you can go for either amd or intel dual core , both are priced around 3k each . AMD 3600+ AM2 or Intel E2140, While not totally necessary it will help
A suitabe mobo from say Jetway is also another 3k
but if you want a absolute minimum cpu amd sempron 3000+ is 1250 at delta, ritchie
or how about u go thru the market section and try to pick up a 2nd rig. it will be really cheap and shud be good enough if you are just gonna run tally,MS office, etc..
even on a minimum budget a amd 3600 is pretty cheap . makes more sense than a sempron ... might as well get some juice that may be used for some applications over time say 5-6 years ...
That would most likely be the Asus M2V-MX. I guess there's nothing wrong with a Via chipset at stock speeds, however i would *prefer* a nForce 6100 or ATI 690G chipset.
Also the Asus lower-end boards have a nasty reputation for poor quality control - just my opinion. A Gigabyte or a Jetway board based on a 6100/690G chipset should come in for around 3k. That would mean roughly ~4.2k for the combo with the Sempron - a mere 300 bucks extra for a better chipset, possibly hassle-free board.
And to answer your earlier question - yes, the Sempron is very reliable, has decent performance and has very low energy consumption/thermal output.