There have been four major releases of the Symbian 60 OS used for the Nokia phones up until now: "Series 60" (2001), "Series 60 Second Edition" (2004), "Series 60 3rd Edition" (2005) and "S60 5th Edition" (2008).
Below you can find some examples about which systems are used on which phones. So if you are unsure about which Nokia theme you need, you can first have a look below:

S40:
5300, 6126, 6131, 6133, 6233, 6234, 6270, 6275, 6275i, 6280, 6282, 6288, 6300, 7370, 7373, 7390

S60 first edition:
3600, 3620, 3650, 3660, N-Gage, N-Gage QD

S60 second edition:
6600, 3230, 6260, 6620, 6670, 7610, 6630, 6680, 6681, 6682, N70, N72, N90

S60 third edition:
3250, 5500 sport, E50, E60, E61, E61i, E62, E65, E70, N71, N73, N75, N77, N80, N91, N91 8Gb, N92, N93, N93i, 5700 XpressMusic, 6110 Navigator, 6120 Classic, 6121 Classic, 6124 Classic, 6290, E51, E63, E66, E71, E90 Communicator, N76, N81, N81 8Gb, N82, N95, N95 8Gb, 6229, 6650, N78, N79, N85, N96

S60 5th edition
Nokia 5800 XpressMusic, N97