I've remembered the RAM: 6GB
For April Fool's funnies, how about the following:
Overspin your hard drive! "Not only can a WD Raptor drive do 10k, so can your common low performance GreenPower drive too! Just..." and here you describe some dodgy mod to do this using hardware/soldering/software/firmware/whatever. Just don't make it so specific that some muppet ruins his hard drive!
SLI/Crossfire a single chip graphics card: another dodgy mod, this time unlocking the "hidden" part of the chip to double its capacity. So, your NVIDIA chip with 240SP's and 512-bit bus suddenly becomes 280SP's and a monster 1024-bits in SLI/Crossfire. Heck, for an extra laugh, you could apply SLI/Crossfire to an AGP card.
Double your RAM: just solder some secret pins on the module and program the SPD.
Run out of motherboard SATA connectors? Add another to the SATA cable, just like on IDE! Another hidden feature...
Enable OverPower on your PSU? No prizes for what that does.
The list goes on and gets too ridiculous for words.
I've wanted to do stuff like this for ages, but don't have the web skills right now to pull it off. Perhaps we can work together on doing this? I'm more than happy to provide the ideas and writeups and you can create the website. Feel free to PM me whenever about this.