Be very careful to browse the forums first before buying RAM if you plan on using it in your DELL. They're nortorious for not supporting certain latencies/brands.
Also DELLs are a completely different form factor to the norm, they are their own. So you would not be able to reuse your case and therefore power supply either/
On the upgrade front:
I'd look at:
Asus P8Z77-V
Intel Core i5-3570K 3.4Ghz
Corsair Vengeance 16GB (2x8GB) DDR3 PC3-12800C10 1600MHz
The Q6600 was a good CPU, but it's flagging now. The 3570K is under £180, and with performance like this in comparison with some of the bigger chips, it's a bargain