You can put this file in your cc_config.xml in the BOINC directory:
<fetch_minimal_work>0|1</fetch_minimal_work>
(Fetch one job per device (see --fetch_minimal_work). List-add.pngNew in 6.11)
Remember to tell BOINC to re-read config files each time you make a change. Otherwise the changes won't be picked up until the next time you restart BOINC.
That way you will get only one task at a time. It will give you time to suspend the project after attaching, so you can add an app_config.xml. Also, no, app_config.xml does not effect the amount of work your client normally asks for. For example, lets say you have an 8 thread machine, but allow a project to use only 2 threads. So BOINC *should* be smart enough to know to ask for only 1/4 of your normal queue size. This came up on the mail lists a while back. DA said that maybe they will address that later. Anyway, let it download however much it wants. It will auto abort un-started tasks if/when they go past due.
Here are all the config file settings, for both cc_config.xml and app_config.xml:
http://boinc.berkeley.edu/wiki/Client_configuration