You should be fine. In a test rig with a Core i7 920 @3.3GHz and the HD 6570, total power consumption worked out to 197w when playing Crysis. Your system should have lower power requirements. Additionally, Dell generally under rates their PSUs as a safety measure. The PSU can probably produce 5% - 10% more power than what it is rated for.http://www.anandtech.com/show/4278/amds-radeon-hd-6670-radeon-hd-6570/15
The Radeon HD 5570 is a safer bet if you are worried.