How many amps can I draw from the 5v on the Adurino Duemilanove?
No more then 1/2 amp max, maybe a little less.
Lefty
It depends how the board is being powered. If it is from the USB lead then you will not be able to draw more that half an amp, even then some computers, laptops in particular will supply less.
If powered from an external power supply it is much less arround 200mA,
for a discussion see:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html
On the standard Arduino board you can actually only draw about 100mA from USB. This is the maximum allowed by default in the USB spec unless the devices requests and is granted full power (500mA) by the host. This is intended to allow unpowered hubs to limit downstream draw to avoid exceeding the total power available from the upstream USB port. The FTDI chip can be configured to request full power (but there is no guarantee that the host will grant it, ie if you've got it plugged into an unpowered hub), but it is not so configured 'out of the box'.