Hi Mahi,
Acoording to the tech specs for this, IF you indeed have the HP DC7600 CMT minitower, you can only use a MicroATX form factor motherboard, and the longest graphics card you can use is 10.5 inches without cutting out the lowest HDD drive bay..
Go to the page marked as "2-3"
http://static.tigerdirect.com/pdf/c00383428.pdf
Defintion of MIcroATX form factor:
microATX - Wikipedia, the free encyclopedia
Now, if you are handy, you can reuse the case, instead of spending $50 on a new case, you can use a Dremel and a free online template to cut out a (92 or 120 MM) side vent or top vent on the case. Plenty of tutorials out there for this. Here is one:
[TUT] How to cut a 120mm / 12cm fan hole
Think of it as, you have to spend $50 bucks, you can either buy a case ,
or reuse this case, and then buy a Dremel and a cheap metal file which will last you for many years for many projects.
I personally would reuse the case. I don't give a **** about what my PC looks like. Every single time we've played together I've been using my PC case as a foot rest. I am cheap, I like modding stuff and I like not throwing out perfectly good stuff.
P.S. when using Dremels and meatl files to mod cases, make 100% certain all the metallic case shavings are cleaned out, or they will short out the motherboard.
EDIT: I see no reason to buy a ATX mobo unless you have a large number of physical drives attached to the motherboard.