Seagate is good, but I've always gone with Western Digital drives and never had a problem. They're VERY reliable and the best in my opinion. Price-wise, they're about the same as Seagate too.
I just put a 160GB WD drive in a computer I built for my dad last week, cost $52.99 from Newegg. (weekend sale)
Just be sure to get the right kind of connection, although there ARE adapters. Your 40GB drive is most likely an IDE connection and your mobo probably doesn't have the faster SATA drive connections, so make sure you buy an IDE drive (slightly cheaper than SATA drives anyways).
I have a WD 120GB drive in a PC I built myself nearly 3 years ago and have had ZERO problems with it.
Buy from Newegg, they're awesome...cheap, FAST shipping, GREAT customer service...
Don't buy from a store like Staples or Office Max, you'll pay double.
I'd get THIS one:
Newegg.com - Western Digital Caviar SE 160GB 3.5" IDE Ultra ATA100 Hard Drive - OEM