BitComet is lighter on resources than most BT clients. It's worth a try.
Why BitComet?
Completely new core written in C++, stable and fast, very low CPU and RAM usage. (detail)
Multiple simultaneous downloads, ability to selected download in one torrent. (detail)
Intelligent resume and seeding, no long-time scanning at all. (detail)
UPnP Auto Port Mapping, no need to manually config NAT in most case. (detail)
Ability to limit the upload speed as well as download speed. (detail)
Simple-look but handy interface, no need to learn how to use at all.
Disk cache mechanism, decrease the potential damage to the hard disk when high-speed downloading. (detail)
Ability to add .bc! file extension for unfinished files.
Only one listening port is needed, no matter how many simultaneous downloads. (detail)
Support Muti-tracker torrent format, and utf-8 extension
Auto Update Checker. (detail)
Muti-Language supported. (detail)
No need to install, but install/uninstall packages are provided.
http://www.bitcomet.com/