I have a Dell Dimension 8300 P4 3.0. We have been

First off, stick to warranty service if you have the option.

While most cases are modular and will accept boards of many standard sizes, Dell cases are made specifically for that model of Dell and are often unusable for other size boards.

Your question doesn't make it clear if the board is too big for the case, or the case is too big for the board. If the board is too big, you need a new case. If you are looking to save money, I recommend geeks.com .

You're venturing into the realm of building an entire new computer - I hope youre ready.

Many of the components from your old dell won't work with a Core1 Duo board. Any of the expansion cards and drives probably will, but you'll need new memory, a processor, a heatsink and fan for the processor, case fans, a new case, and probably a bigger power supply.

If you aren't comfortable with thoroughly educating yourself and investing time and money into building a new computer, you're better off buying a new Dell or other premade computer. If you're brave, look at tomshardware.com for info.

Good luck!