installation is just plugging the card into a slot (some cards need a connector from the power supply,) and then putting a CD with the software into your computer. It's a pretty easy process.
The card I'd point you to depends on the budget and what games your brother is looking to play.
For light gaming like Sims, SimCity2000, Doom (and other old stuff) you'll probably do fine with the ATI HD4650
(~45 pounds) or something even cheaper
For general gaming, something like the ATI HD4830
(~60 pounds to 95 pounds)
For heavy gaming (stuff like Crysis on a 1920 x 1200 monitor) you'll want something like the ATI HD4870x2
all those are ATI, Nvidia would do just as well...but they rename their products so often that it's hard to keep up with what is what.
You'll need to know what kind of interface the motherboard has (this
and this other thing
and another other thing
If your brother isn't planning to buy all the parts and build the computer himself chances are that you won't be able to upgrade the graphics card too much as the more powerful cards need 6-pin or 8-pin connectors (or two of one, or one of both) which something like a typical Dell isn't likely to have