With such a wide range of power tools it is sometimes hard to decide what tools you really need. Fortunately there are three common tools that are capable of most tasks - Drill, Rotary Tool and Jigsaw.
I try to stick with corded tools over cordless as batteries have a nasty habit of not being charged when you really need them. Remember that most metals used in a majority of cases aren't very thick, so you don't need the most powerful and expensive tools on the market.
Drill - $40
Most people will have a drill already, but if you don't and are planning on buying one, do some research first. When purchasing a drill you first need to decide on corded or cordless, remember to check each units drilling capacity through desired materials.
Drills are useless without some drill bits. A basic set will suffice for most people, but for more extreme mods like adding large case fan holes a set of Hole Saws may come in handy.
Rotary Tool - $40
Probably the most commonly used, multi-faceted tool in any modder's toolbox. With the ability to cut, polish, sand, file, etch and countless other applications the rotary tool is used in almost every cutting or grinding related task. Some people confuse the name of this tool with Dremel, which is in fact just a brand of rotary tool.
Rotary tools come with an almost endless range of attachments and extras and you may find yourself a bit overwhelmed. I usually just purchase a rotary accessory kit when I need to stock up as they tend to be a bit cheaper and are an easy way to stock up on multiple attachments at once.
The only recommendations I have when it comes to rotary tools are choose one with multiple speeds, this will allow for a much wider range of applications. Also when purchasing cutting discs it is usually better paying a bit more and getting diamond cutting wheels. Diamond wheels are a lot more durable and will save some time and frustration with standard broken discs.
Jigsaw - $50
The most practical tool for cutting case panels and acrylic the jigsaw has some very diverse uses. Whether cutting out venting, creating a complete window modification or with some practise cutting out fan holes, a Jigsaw with the right blade will get the job done. When purchasing a jigsaw check that your preferred saw can handle cutting the desired materials and thickness's that your mods may involve.
Jigsaw blades come in many different types for cutting through different materials. I like to have blades suitable for plastic (acrylic) and metal at hand at all times.
Sander - $50
Palm or orbital sanders are not an essential power tool, but are good for lazy modders or larger sanding jobs. You can pick a good name brand palm sander up for around $50 - the last tool I purchased was a great little palm sander. I was amazed by the features for the price; it included a dust filter and a micro corner sanding tip making it perfect for even tiny sanding tasks.
Palm sanders have a large variety of different grit sanding pads available. Pick your grit and sanding pad size according to the job at hand and the brand of sander you own.