The Grand Theft Auto games always get boring once everything is done. I usually go back to Gran Turismo after a few weeks. On my new PS2 (had it for over a year now) Gran Turismo has occupied over 75% of its life (no joke - the rest is occupied by GTAIII and VC, NfS: HP2 and a couple of games i rented out).
In the last year i've only rented out about eight or less games and bought only two (V8 Supercar race Driver and Need for Speed: Hot Pursuit 2).
Only plan on buying three games this year (none so far) those being Prologue (11 days), GT4 (November) and GTA: San Andreas (November). Don't plan on renting any more out (well, maybe Jak II and Broken Sword: The Sleeping Dragon to get them finished).
Anywho...