It isn't the service that you add that is going to determine the load on the Rpi, it is how CPU or I-O intensive the application is.
DNS and DHCP is neither CPU or I-O intensive so unless you have an errant client asking for name resolution or a new IP a few thousand times a second, there will no impact to the Rpi.
Samba shares can be very I-O intensive so the Rpi might not be able to keep up as the I-O bandwidth of the Rpi is limited.
In short, the Rpi is a handy device, but it isn't a General Purpose server either. A Dell 690 could handle several thousand busy email accounts on a Dell server. I doubt that a Rpi could handle more than 50 or so.
Doing Unix since 1985.
The 9-25-2013 image of Wheezy can be found at: