The 1st gens can only work on the 2.4GHz band but can still extend that network - but you will lose speed firstly as any extender does just for being an extension, which halves the speed and secondly as it is limited to 54Mbps as well so it's a double whammy.
Only extend if you specifically need the coverage or need to use the extra functions for AirTunes, a printer or a disk.
My suggestion would be, if you need the extension for whatever reason is to get new Airports - eBay is good for this (other sites are available) and sell your old ones - they still fetch a fair price second hand so in the end it doesn't cost much. I upgraded just such a configuration by buying two newer APs and sell 3 old ones - - actually made a profit on that!
Regards,
Shawn