to check on your power supply, you would have to open the case and look on top of the PSU, it will probably be a 235w supply as most older time/tiny cases were (i think).
this would be ok for a basic system running AMD up to about Athlon 2000XP, but if you plan to get a new graphic card/ cd rewriter/ 2nd hard drive etc then the power supply might not be good enough.
most atx boards will fit your case, the I/O plate may need changing on the back but they normally just pop out and in easy enough.
i have just built a system using an Asus A7V8X-MX motherboard £40 and was pleasantly suprised by the board, it has onboard VGA,SOUND,LAN and 4 usb 2.0 ports (and a header for two more)
open up your case, have a look and see, if you have doubts then get a new case as well (they start at around 20 quid). or if you are happy with the case you could but a bigger power supply.