Dell XPS 420 800MHz memory - worth upgrading to 1066MHz?

No. The cpu fsb may be at 533; this doesn't mean the ram is running at this speed. Cpu and ram fsb aren't the same. Dell systems aren't designed for upgrading. If you want to upgrade, build it yourself.

Not sure I understand bu thanks anyway

^+1 No. The difference between 800 MHz & 1066 MHz RAM could be from a small LOSS to a small gain; it is NOT 20% gain and at best a 2~5% gain. It's all about the IC's used in the production of the RAM.

Example:
Comparison Module #1: Mushkin Redline 996805 6-8-6-24 1600MHz
Comparison Module #2: G.Skill Pi Series 7-8-7-24 1600MHz
Comparison Module #3: Mushkin 998687 8-9-8-24 1866MHz
Comparison Module #4: Corsair Dominator TR3X6G1600C8D 8-8-8-24 1600MHz
Comparison Module #5: Kingston HyperX T1 9-9-9-27 1600MHz
Comparison Module #6: Mushkin Blackline 998677B 7-7-7-20 1333MHz
Comparison Module #7: Patriot Viper II Sector 7 9-9-9-27 1800MHz
Stock Speeds:

Thanks Jaquith! That's really helpful!