The Custom Firmware adds a lot of extra functionality, I suggest you have a look at the WiKi HERE. There is a list of features HERE and some sample screens HERE. There are some differencies in the way custom firmware is loaded and used between the HDR-FOX T2 and the HD-FOX T2 so look out for them, e.g. HERE
The basic functionality of the box is all contained within a single binary 'humaxtv' and this has not been changed in the Custom Firmware. Therefore there are no changes to HDMI or the standard Humax UI. The main changes are the hooks which allow additional programs to run to provide extra functioanlity (e.g. webif, remote scheduling etc.).
I would be a bit wary of using an external USB HDD without a power supply. There have been some reports of problems with this arrangement.
There is a Linux program that runs on the box called humaxtv, This program has not been changed so there will be no change in HDMI / TV picture quality, Extra packages can be loaded to run along side the main program to add a Web Interface that you can log into from a P.C. Although the custom Firmware is best loaded from a flash device in exactly the same way as you loaded the standard .20 firmware, After the custom Firmware is loaded the HD-FOX T2 needs a Hard disk that has been formatted by the Humax box, So It would be best to get your 1TB HDD up and running before loading the custom firmware from Flash
The Humax USB will supply power for an external HDD the same as a P.C. would, However it doesn't supply as much power as a P.C, Because of this some USB powered HDD's don't work with the Humax, However if your 1TB doesn't work it is possible to connect it to a separate power supply. Other tips / Fixes / Bugs HERE
Note, Your Humax is formatting the HDD in a format called EXT3, This format can't be read by a P.C. (without adding extra programs), However this isn't a problem because you can get files to and from your 1TB using the LAN network socket, Where your Humax and your P.C. are on the same network
Installed the web interfce. Thing is, my router is downstairs, and the box only has one USB. So i cant really be connected to the net and have my HD connected at the same time. Is there any solution for this?
Edit: tried installing packages via LAN but doesn't seem to work i dont think. Example
It will have finished installing them - some packages do cause the web interface to get stuck just after installation, just click to another page.
You can install packages via a USB disk if you don't have a permanent LAN connection - see http://wiki.hummy.tv/wiki/Manually_loading_Features_from_USB but that still requires you to unplug the hard disk. It really is worth getting a LAN connection up and running - Wireless works fairly well with a £10 adapter or the more expensive Home Plug option is what a lot of people use.
Your Hard Disk Drive can always be re-formatted to a different 'format' but anything on the HDD will be lost, Also keep in mind that your HDD will have a section reserved for the Custom Firmware files which would also be lost. If you get your Humax Networked you can leave the HDD connected to the Humax and FTP files in and out of the HDD so the EXT3 format is not a problem, FTP'ed files don't retain their EXT3 format when transferred to your P.C.