Keep the hdds and dvd drive and possibly the OS if it can be transferred and start anew. Maybe see if you can resell the rest on CL? If the OS cannot be moved to a new system, then try selling it on CL with one of the smaller hard drives so you can recover some money.
While it would be nice to use the case, those prebuilt comps from dell and the like have special cases that really won't fit most equipment you want. A cheap case though will only run you $30 or so and provide superior airflow and decent enough looks in comparison to the dell.
So basically, need
cpu and mobo
ram
gpu
psu
case
Hopefully you won't need the OS or you can get it cheap through a school if you are a student. If not, a prebuilt customized one from ibuypower may be the best option.
You are probably looking at $40 on ram, $30 on case, $100 on cpu, $100 on mobo, $40 on psu, $200 on vid card. Should be an impressive little system. If you can up your budget any more or if you happen to live near a microcenter, you could even snag a top of the line intel cpu with mobo bundle at a hefty discount.