The best rule of thumb for buying gear goes like this:
1) What are you trying to accomplish with your recording setup? Personal demos? Recording other people? Full band or one instrument at a time? Etc.
2) What is your #1 problem right now with your setup?
For example, if your mixes aren't translating, then monitors and treatment are avenues you should explore. If you're interested in doing full band recording, but are running out of inputs, well then you know what you need to do. So on and so forth.
The bottom line is, ignore what everyone else says and take stock of your personal situation and your upgrade path will be clear. Then just do a ton of research on different products in the area that you've identified and make yourself as informed as possible.
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Monitors and room treatment. I can make a great mix on a free audio program with a Mackie console if I have a decent sized room with bass traps, treatments and a pair of great monitors.
