Well there is a current Dell vs Asus battle going on ! The benchmarks i've seen put the Asus MyPal 620 as the fastest so thats what i bought.
However the Dell is a very good machine and is cheaper but to me speed was #1. I've got the Asus and i can now run Mame games full speed, SNES games full speed with sound, Genesis and GBA games mostly full speed (some are still a bit too slow)
With a 512 MB CF card i have a ton of games on it and the MyPal is superb.
Unsure about how to convert the roms though. Anyway i'm pleased as anything with the Asus and the controls are very good