I had that problem and this is how I solved it...
Install "nvdock" from synaptic and open the program - Start > system > NVdock. It will load in the system tray. Right click on it and it will open the program.
Look in the left side of the window and click on the listing that is at the bottom and see if you see "Overscan" in the main window. If it is not there click on the next to last listing on the left, and go up until you see it. I think it is in the next to last, or the third from the bottom. When you see it, click on the slider bar and drag it to the right. It will resize the resolution. It works like a charm on my HTPC and I searched a long time to find it.
Hope this helps.
Desktop: AMD 3.6ghz X4, Gigabyte MB, 8GB ram, Multiple PCLOS's, 126.96.36.199-pclos3.pae.bfs
Server: Intel 3.4ghz X2, 4GB RAM, SATA, gnome2, *.pae.bfs, Headless, Servers: LAMP, FTP, Samba, Print, PXE, BT
HTPC: Phenom II X4 2.8ghz, 4GB Ram, 1TB SATA, PCLinuxOS-KDE, NVIDIA N220GT HDMI, *.pae.bfs