I have an i3 and also haven't figured out a good place to mount the power supply. I did install a set of Molex 0.093" connectors to get the two separate 12V rails from the power supply so it's fairly easy to detach the power supply if I need to move the unit.
As far as your choice of power supply, a word of caution: I started out with a 400W power supply which appeared to be rated for 20A output on the 12V rails but actually didn't come close to that. I know a 450W power supply is specified for the i3 with heated bed but after scratching my head trying to figure out why I couldn't get both the heated bed and hot end up to temperature for ABS (110C and 230C) I got a $47 Antec Basiq 500W power supply from Fry's. Now no problems getting quickly up to temperature.
The important points here are that
1. the failure mode from having insufficient current from the power supply for heating can be that it just takes too long to reach temperature, and you get significant drops in temperature during printing (which lead to jamming). Taking too long to heat can also lead to burnt residue in the hot end nozzle.
2. Cheap off-brand ATX power supplies may work for most PC replacement purposes (even if you have a power-hungry graphics card) but don't always perform up to their rating. Better power supplies will come with a diagram which tells you which lines provide the separate 12V rails. Usually the 4-pin PCI express connector (yellow and black) will be on a separate 12V rail from the 12V going to the 20 or 24-pin mainboard connector but there's no rule for that.
RAMPS 1.4 with Marlin has the option for PWM control of a 12V DC fan when cooling is needed, otherwise you probably don't want to cool down the print bed. If you measure the temperature on the glass (top) part of the print bed with a thermocouple or IR thermometer you may find that it's already quite a bit cooler than the thermistor. Extra cooling fans are pretty cheap (USD $5 to $8).
