Even if you're skimping on upgrades, I still highly recommend that you use the following upgrades or at least try them in games.

StimPack - This will greatly enhance the attacking abilities of your marines and firebats. It's essential if you rush with these guys or have to defend against a devestating attack. Remember that this will also impair the marine/firebat that uses it.

Ghost Cloaking - The enemy tends to target your ghosts first, and if you need them for an important task, you'd better protect them!! Cloaking is the simplest way to do so, but you'll use up your ghost's mana for every second it remains cloaked. You'll end up with less mana available for Lockdown.

Wraith Cloaking - I don't really like Wraiths that much if you cannot cloak them! They are pretty much useless then. I don't always use cloaking to take enemy units by surprise; sometimes, I send cloaked Wraiths to scout. The enemy won't even know that I discovered their expansion and neglect to build a significant defense there. So I'll attack it immediately and they'll really wonder how you found him.

Yamato Cannon - This is one of the most awesome weapons in the game!! For 150 mana, a battlecruiser can shoot one shot with this weapon. I think it does like 300 damage but I'm not sure.

EMP Shockwave - Only research this if your enemy is Protoss!! A SCI vehicle can disable Protoss units' shields with it and they'll be very weak.

Psionic Storm - It can do massage damage over an area, so it may determine your victory or loss in a battle. It works even better against the Zerg; cast it on their buildings!

Upgrade Air Weapons - Very important if you have carriers. Each interceptor can do 1 addition point of damage each time you upgrade air weapons. If you have 8 interceptors ... 8 more points of damage per carrier!

Increase Carrier Capacity - It allows you to make a maximum of 8 interceptors instead

Zerg

Burrowing - This is probably the most important upgrade for the Zerg. It affects ALL Zerg units except for Ultralisks and air units. You'll wonder how you coulda survived without it if you face the ordeal of one of your expansions being attacked when you immediately burrow all your drones and send in reinforcements to save the base. :)

Ventral Sacs - You'll need this cuz it'll enable you to transport units with overlords. There's no other way to transport other than with overlords.

Pneumatized Carapace - Overlords are way too slow. It's obviously a good idea to make them faster since they also also so useful for transporting, scouting, and detecting cloaked units.

Muscular Augments - Hydralisks are rather cheap and cost-efficient but they are so slow. Better make them faster!!

Spawn Broodlings - This spell is the bomb!! It'll destroy any unit and replace it with two broodlings. Overall, I think it's about as good as the Terran's Yamato gun. However, it won't work on buildings or robotic units.