I've read that it is not advisable to reuse the old kernel's configuration file unless it is just an upgrade from a kernel revision to another, ie 2.6.19-r1 to 2.6.9.19-r2. Apparently this is due to too many changes from one kernel release to another (Source: Gentoo Documentation Guide).
So here's a list of stuff that I should remember to edit when compiling a new kernel:
1. Change the processor to Pentium 4
2. Add NTFS support (I dual boot)
3. ATI open source drivers (as modules)
4. Sound modules
Will update the list whenever I remember something else