For the Gamers:
ZOTAC GeForce 9800 GT - $88
The main bottleneck in most gaming PCs is the video card, or rather, the lack thereof. Many computers, either store bought, or even from specialized gaming PC vendors, use either a low end video card, or even worse, integrated graphics. This severely impacts the computer's performance in games, to the point of rendering most new games unplayable. Fortunately, there is a cheap solution that will give you great performance for your dollar: The 9800 GT Video Card. This card can be found for as low as $88, and offers enough performance to play any modern game at high settings. Even better, it doesn't require and extra PCIe power connector, making it a viable option for most store-bought systems that come with weak power supplies.
Powercolor HD 4650 - $50
Maybe you're not a huge gamer, but still enjoy the occasional strategy game or shooter. The HD 4650 is a great, cheap add-in card that can give any PC the ability to play new games, an ability that onboard graphics just can't deliver. This card draws very little power, so it can run on virtually any PC without other upgrades.
OCZ 4GB DDR2 1066 RAM - $48
Another limiting factor in many PCs today is the amount of RAM they have. Most PCs that are bought from a store come with 2 or 3GB of RAM. The problem is that the combination of modern games and the Vista operating system can easily use that much RAM at any time, causing slowdowns. RAM is one of the cheapest components to upgrade, fortunately, and you can now get 4GB of fast, DDR2 1066 for as little as $48.