I would go here
http://forum.xda-developers.com/showthread.php?t=684049
and read on from there......
as far as overclocking goes yes you can. some people have roms that have that option available. just check the section where the roms are. there's even 1 user who is porting over 2.2. so just look around there, register if need too.
as far as task killers. they do close programs. i had at 1 point as much as 95mb free. i noticed with the 2.1 update there were a lot more programs running then there was when 1.5 was installed.