As Bob_B already mentioned , Dell PCs come with a Dell-centric version of Windows. Changing the motherboard will invalidate the Windows product key so then you're stuck with paying for a regular copy of Windows and installing it from scratch on a reformatted drive.
If by some chance you have a spare copy of Windows available that's not in use elsewhere, then you'll need to do a little homework to choose a motherboard. This is because some LGA 775 boards support DDR3 RAM while others are restricted to DDR2. So, you need to confirm exactly which type and speed of RAM your brother has, assuming you don't want to pay for new RAM. Next, you must go to the board maker's website, click on the support page for the desired board, and read the user manual's BIOS section to see if the cpu multiplier and/or base clock is adjustable for overclocking. Usually, a Dell PC does NOT offer these adjustments in the BIOS. My GF's Intel DG33FB is a very stable, reliable motherboard but, again, i don't recall seeing any BIOS option to adjust B-clock frequency or Multiplier. Don't forget you may also need to slightly increase the cpu vcore voltage and, probably, replace the stock intel heatsink/fan with a superior aftermarket cooler. So, check out some available boards and their user guides which usually include a page-by-page description of available BIOS settings. Here's a link to dozen boards available online:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007627%20600007879&IsNodeId=1&bop=And&Order=RATING&PageSize=30