OK, that's a lot more to work with!

Gaming will be the main factor here - with the video viewing coming in second place.
So, the board has to have AGP.
He can get by with 512 mB of RAM for some games, but a lot will work better with 1 gB - some games even take advantage of 2 gB now! So how much RAM is the next question (and what type? DDR2 is better for gaming - but it comes at a price). Most boards these days will accept at least 2 gB - so that shouldn't be an issue for now. But, as with the other technologies, DDR RAM will become more scarce as DDR2 gains in popularity.
In order to preserve your option to upgrade to a dual core - you'll have to not only get a socket 775 mobo - but also get one that states it will support the Core 2 Duo. Not all socket 775 boards will support it.
What about a hard drive? Use the old one, or get a new, faster one? What interface would you like to use - IDE or SATA? Depending on the games that he plays, and the music that he downloads, he may need more storage space in addition to a faster drive. Luckily there are cards on the market that allow you to convert to SATA later on (if you want to) - so you can safely use an older drive and then upgrade to the SATA later on if that's what you want.
After all this I went to
http://www.pricegrabber.com and did a search for Socket 775 boards with AGP. There was only one available - and it didn't support the Core 2 Duo.
So, I went to
http://www.pricewatch.com and found more - but I can't vouch for any of the sellers there. On the first page #'s 2, 5, 6, and 7 support the Core 2 Duo. Here's a link to the search there:
http://www.pricewatch.com/motherboards/socket_775.htm FWIW - BioStar makes a good motherboard but it doesn't have a whole bunch of features. FWIW - I'd select a board with an Intel chipset, an nForce chipset, or a VIA chipset.