Well it really depends on your particular area is, what your specialities are and your personal preferences. Personally I wouldn't be without SuSE 9.1 Linux running Apache 2, Open SSL 0.9.7e, MySQL 4.1.7, php5 and whatever html editor. I use Bluefish myself. On hardware somewhere around 2ghz cpu with 512 - 1024mb ram and a 40gb HDD, or better specs. Around about that though, y'know. You could easily go plenty higher or lower depending on yer budget (thankfully the total cost of the software would be fook all though.
Whilst that setup is ideal for me though, it'd be completely useless to many others. I have no intention of using Flash so any old html editor will do me fine and the OS is ideal for rolling out the latest versions of Apache, OpenSSL, MySQL, PHP and so on with no hassle. Whereas with a windows platform you are highly likely to run into some difficulties running them all together in exactly the way you need. The same would be true of trying to run a lot of MS stuff on Linux too. Portability is much better than it used to be, but I'd still much rather run the programs on the OS'es they "belong" on.
Not sure if C (if that is of any concern to you) is any easier to use to write applications on one OS or another though. My friend says he's having considerable trouble with it on Windows, but I've no idea if he'd have the same trouble on his Linux too. He hasn't writing any C on his Linux OS yet.
If you tell us which areas you specialise in, I'm sure some of our more experienced and knowledgeable members could be of more help to you.