Since you are specifically asking about PE and not about PR, I can't really comment, but had you asked about PR, I would say:
1. Lose the Xeon, get an i7 instead.
2. Lose the Quadro 600 and get a GTX 560+
3. Increase memory from 4 to 24 GB
4. Lose Dell as your supplier and either build yourself or get an ADK machine.
Look here what makes a good machine for PR CS5+: PPBM5 Benchmark
