900hp Fiestas? Why?
I guess I'm a little different than the average player, in that I don't go for big power mods. I try to keep cars within the class that they originate in and see if I can improve them by just tweaking the chassis set ups. Stuff like camber, toe, roll bar stiffness, tire pressure, brake balance. Especially in Forza, where you are awarded tons of cars for leveling up and winning races, but they are worthless to sell. So I keep them in my garage until I come across a race series that they fit in perfectly. I think I only have 4 or 5 modified cars in my garage of 30+. I do have a mildly tuned Fiesta, and also an Evo X, an STi, a Hachi Roku, and a 300ZX that have some light engine mods as well as chassis tweaks. The 86 in particular is a hoot to drive against other cars in its class.
Unlike Gran Turismo, where some of the cars you win are worth a ton of cash when you sell them (I'm looking at you Toyota RSC in GT4!). Sure I had 1000hp Mitsubishi GTO's and 800hp Subaru wagons and other such nonsense, but they were pretty terrible to drive, and I learned next to nothing by simply throwing money at every part on every car. It was fun though, for a time. I'll tell you where GT lost me though, and it was nothing to do with the gameplay. It was when it took them 4 years after the PS3 debuted to get the game out. In the meantime, I bought a 360 and fell in love with FM3 and 4.