1st off... and I dont mean to sound like a dick but... just about anything is an upgrade from a Dart.
That being said, you really need to be more specific about what you're looking for.
If you're looking for a fork just like your Dart but 'better', then both of the other posters are actually correct... The next steps up from the Dart are the Tora, then the Recon, then the Reba... There's also the SID which is less all around and more XC dedicated so it cuts the travel a bit but also shaves some weight.
If you want something a bit beefier, the Revelation is a great fork and the Pike is solid enough as well.... There's also the Domain but that's about the upper end of what I'd recommend installing on that frame, anything bigger would be too big.
PS- Of course there's great options from other comanies too like the Marzocchi 44ATA or the Fox 32 series but I stuck with RockShox since you're already a bit familiar with them. If you want some more specific details, feel free to email me.