The i5K is good for OC and so is the board.
I honestly don't have the money. Are there any amd alternatives to the i5?
I believe that AMD doesn't provide much competition for the I5-2500k at the moment.
Something like a Phenom II X4 965 BE might do better than an I3-2100, at about the same price. The chips with more cores (Phenom II X6, FX) might be attractive if you need to run software that can efficiently use multiple cores.
It appears that Dell may use a proprietary connector for the front panel stuff. It may require some extra effort to use an industry standard motherboard in a Dell case, even if your GX280 is one that can fit a standard format (microATX or ATX) board. (Dell made GX280 PCs in a variety of form factors. Which one do you have? If you don't know, take the service tag number to support.dell.com.)
If you want a graphics card, it looks like the best available that doesn't require an auxiliary PCI-E power connection uses the Radeon HD 6670 chipset. I hope that your PSU has enough capacity to support one. Also, that your case can accept full-height graphics cards.
What's your budget? A quck look at
www.newegg.com suggests that you could get a Phenom II X4 965BE (retail, with heat sink/fan), an inexpensive AM3+ motherboard, and 2 X 2 GB of DDR3-1333 RAM, for roughly $200US. A 6670 graphics card would add $65.
Maybe your best course would be to plan and save for a new build. Here's a $500 example:
April 2012 Gaming Computer Builds
You might be drawn into wanting to spend more money, though. If you started with an I5-2500k ($220) and a Radeon HD 7950 ($400), I imagine that you could build a high-end machine for not too much over $1k. That's a lot, but you'd get rapidly diminishing returns in spending more.