What was the 250W, because if it was an Emachines system with a 250W BestTec, they were notorious for taking out the motherboard when they failed.
The other worry with a 250W - is it standard size, as 300W has been pretty much standard for the "anonymous grey metal box" for as long as I can remember, except for smaller format or those awful BestTecs.
PS. The CX430 seems pretty decent, though criticized as not actually as good as the previous CX400 model.
JG has the specs & test
http://www.jonnyguru.com/modules.php...Story&reid=214
Another word of caution when replacing older units with modern, units from the era of "12V main power" will not satisfactorily run systems that had the main load on the 5V, but they are pretty old now.
Also, -5V was deleted from the ATX specification (Optional in ATX1.2, prohibited in 1.3 - 2003) - older boards may require the -5V, though in the main it was to support ISA slots.