650 for a mac mini 1.25ghz, and 1gb of ram is overpriced, IMO. IT should be less than 400 for the actual hardware's value. However, always go for more ram because a little boost in processor speed won't be noticeable compared to more ram. The G4 you are thinking about is much better in value (although still overpriced) but yet again... the mini is very small and portable.
Dual monitors are not possible on the mini.
SilverStone Temjin TJ06
P4 3.00 Ghz Prescott w/ Zalman CNPS7000B-Cu
2GB OCZ Value Series PC3200 (2 x 1GB)
BFG GeForce 6800 GT @ 370/1000
SB Audigy 2 ZS
Antec TruePower 430W PSU
Sony SDM-HS73 17" LCD