In the spirit of that, I thought I'd share how I'm planning out my purchases. It's not the most organized way to do it, but it's the way I've always managed to get enough stuff to construct an entire army.
Usually, I start off with a specific list. Then, I compile a second list of the number of each model in the list. For example:
25 Regular bolter guys
4 Plasma
4 Melta
I think that probably explains it. Then, I figure out how many boxes I need to get what I need.
Of course, this time around I'm not necessarily building towards any one particular army list, so my list of stuff seems really long... Also things are complicated by the stuff that I already have. So, I get 2 lists.
What I have:
IG infantry
29 lasgunners
4 sarges
12 plasma gunners
3 melta gunners
2 sword bolt pistol guys
IG vehicles
4 Chimeras
1 Vendetta
Space Marine infantry
5 Assault Terminators
21 regular bolter guys
3 sarges
6 melta guys
Space Marine vehicles
4 Rhino/Razorbacks
1 Landspeeder
1 Stormtalon
1 Dreadnought
What I have on the sprue:
11 bikes
2 Hellhounds (which will be converted into Hydras once I have the appropriate platforms)
30ish space marines
What I plan on making:
Space Marine infantry
11 more regular bolter guys
15 missile launcher guys
2 flamer guys
2 multi-melta guys
2 lascannon guys
4 plasma guys
5 Thunderwolves
an assortment of HQ choice-esque guys
Space Marine vehicles
2 more Rhino/Razorbacks
1 more Dreadnought
2 more Stormtalons
2 more Landspeeders
2 Predators
1 Vindicator
Imperial Guard vehicles
2 more Vendettas
2 Leman Russ demolishers
So... Yeah. This should give me plenty of options to field lots of different versions of a couple of armies. At this point, that's what I really want. Lots of options = less chance of all of my choices being invalidated by codex or edition change. I'd also like to try and make one of those Thunderhawk conversions that I've seen floating around out there. Perhaps I'll make that a reality, or perhaps not. I suppose we'll see as I move forward.