I wouldn't call myself an expert, but IMO I would go with a Linux server. I use SUN for more hardcore data storage and processing but I use Linux for all of my network services (DNS, NIS, Apache, etc). I don't run a huge web server, but your Dell (I'm not a HP fan, had multiple hardware problems on close to everyone I have ever used) running a version of Linux should be fine. I wouldn't recommend 7.2 though
The newer stuff, for the most part, has bigger/better/faster features and up-to-date security patches.
Solaris is my workhorse and Linux is my front end. Just my $0.02.
Oh yeah - if you are comfortable with the install process, I would re-install anything that comes from a vendor. There are, often times, lots of things you don't care about or need, and most of the time their partitioning (once again IMO) sucks.