I've been programming PIC 18F452 for almost a year and have a pretty good understanding of the chip now. The 18F8520 is so new, I haven't found any development/evaluation boards for it yet.
The MPLABS IDE is a simple IDE, but eventually I switched to Komodo as it works with CVS (concurrent versioning system). For programming, I've been using the Melabs EPIC programmer.
Hi-Tech sells a C compiler for the PIC which will come with a Windows version and a Linux version, but is expensive. C2C is cheaper. The CCS compiler is a popular choice, but isn't fully 100% ANSI C.
No SDCC or GCC port yet, but I'm still hoping this will change soon as the incentive is there. The 18F now has enough of a stack to make this possible now.
Each compiler accesses the PIC hardware with different semantics, so changing compilers means you have to rewrite the code.
As for an IDE, you have many choices besides VI (vim) and Emacs/Xemacs such as Eclipse, Kate, KDevelop, Komodo (ActiveState.com) and Kylix. I probably missed a few, but these are the ones I'm familiar with. Most of these can be extended to support the compiler and a download utility.
For programming a PIC, there's PonyProg and PP06, neither of which I've tried yet as I haven't yet completed my Windows to Linux conversion on my last Windows box.
Btw, check this out:
http://www.penguicon.org
Linux rocks! Windows sucks. Why pay the Microsoft tax on your computer?