@Dominica Time Machine & Migration Assistant ought be the ticket. Quick links—
OS X level wasn't mentioned. OS X 10.5 is very much on its last legs, and 10.6 is close to being unsupported. Generally, am reticent to move a Mac model too many OS X versions away from the one it shipped. Newer OS X levels work better with more RAM. Apple "lies" about max RAM capacity, check MacSales/OWC &/or EveryMac for prospective maximum RAM of the precise model (usually found by cross-checking the serial number).
If the Mac is older, say 3-4 years old ... it may be time to swap out the hard drive for something newer. Just a general tip which has served well over the years. (Newer is also a big bigger and prospectively faster.) If swapping HDDs then Time Machine becomes moot, as that HDD can be dropped in an external enclosure for Migration Assistant to transfer applications, data settings, accounts, et al. After the migration is complete and a break-in period passes (30-90 days according to comfort level), that "old" HDD can be used as a Time Machine HDD (being old & approaching suspect, but not "bad" ).
Caveats which come to mind—
- Ought be the first thing done after wiping the machine and the reboot after the OS install finishes
- New OS X versions have improved Time Machine (e.g., 10.7 or 10.8 multiple Time Machine backup points are possible such as one HDD at home & one HDD at the office)
- Use a HDD enclosure with the best interface possible (FW800 or USB3), mainly as waiting stinks (many GBs will take a while).
- If mixing OS X levels, some old programs may not work (well, yeah, no longer compatible & all that)
- Encryption/Filevault will complicate things (either on TM or the boot HDD)
See >> Simple!!
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Believe KatieM has Mac expertise, and has a few tips & tricks to note, of course.