On making lots and lots of NPCs? In my experience, it's absolutely useful. Make dozens of NPCs. Make hundreds! Just not all at once.
The secret I found for DMing 3.5 in particular? Never throw away a stat block. If I make, say, a 9th-level assassin for the PCs to fight, I keep that assassin's sheet on file. Also, with the right tools, it's not particularly hard to level things up and down a bit while creating the blighter in the first place, so I may make a 6th-level version, and a 3rd-level version, and a 12th-level version for a couple minutes extra work, and I can keep those on file so that if later, I need an assassin to kill someone because a conversation is getting really boring, I can pull out an appropriately leveled assassin with a few simple, cool tricks on the fly.
This mostly depends on DMing style, though. I don't do much planning, so my ability to DM depends largely on my ability to pull stuff out of the Big Binder on the fly, and to put the sorts of things I'm gonna need in the Big Binder in advance.
The way I handle this in D&D is I have my players write down the relevant checks and saves on an index card, then I do the rolls myself. I also keep a stack of index cards on my right and use that to inform the players of what they have seen or heard.
Lets say I have four people in my game and somebody is hiding, I flip through their index cards and find their modifiers to the spot check (4, 6, 3, and 8 for example), and I roll a 7. I apply their modifiers to the roll of 7 to see how each player did (11, 13, 10, and 15), and then let them know if they saw anything (either via more index cards or as an announcement depending on the importance of the situation).
The problem I have with this (and, by extension, things like passive perception) is that it's always the same character who spots everything. If anyone in the party spots anything, the elf Ranger spots it. I do a similar method, myself, but I keep multiple rolls (color-coded dice/index cards helps), so that sometimes, the Fighter with no ranks in Spot and a +1 Wisdom modifier might see something the Ranger missed. It's a minor quibble, but it's something I don't like losing in games.