Yesterday I installed xubuntu ( http://www.xubuntu.org/ ) on an 800Mhz/ 256MB PC successfully. RAM will be much more a limitation than clock-speed, so getting 512 MB or 1 GB would be a good idea, some money presumed.
Most linuxdistros would be installable, but a lean one like xubuntu seems most suitable, while a super-lean-one like DSL isn't neccessary.
Since you already have WinXP installed and you just want Linux/Solaris for experimenting, have a look at VMware Player. It can run various OS's within XP. VMware has preconfigured Linux and Solaris virtual installations available.
The latest OpenSolaris also runs quite nicely in a Xen VM.
Both X86 Solaris and Linux are free. To get industrial-grade support for either, you'd normally want to send someone money, be it Sun, Red Hat, SuSe or the Ubuntu folks.
As someone who's been involved on both platforms for several years, I'd say that the overall structure of Solaris is more like that of Unixes back in the early 90's. The commands have fewer options and the /etc tree may have some binary stuff in it - /etc is essentially all text files in any recent-vintage Linux.
What Solaris lacks in polish, however, it makes up in workload capabilities. It has some major functionality designed for heavy-duty load balancing. Also, the Solaris Zone feature is pretty neat. It's more or less a step between simple chroot jails and the multiple-OS capabilities of Xen or VMWare.
And yes, things like bash are available on Solaris. Also, although the basic Solaris package set isn't as rich as it is with most Linux distros, you can find lots of goodies at sunfreeware.com.
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