Really depends- if you are looking to go 100% than water/liquid cooling is OK- however I've heard (and can clearly see) it isn't easy.
It also depends on what system revision you're using- an original (with the 30% failure rate)? Get a 3rd party if it isn't already dead. If you got any later models you may be OK with the you got now, as long as it fits into the case you are using.
360 Slim? The standard one should be fine, and you shouldn't be too afraid to try a high end laptop heatsink (especially old Pentium 4 era Alienware laptops!). Those are the least prone to overheating and so using a different heatsink shouldn't be a problem, keeping in mind you have
-the heatsink mounted properly
-the heatsink thermal capability at or above the thermal load of the Xbox (not sure what it is)
On the note of thermal paste- I've never used that stupid Arctic Silver what-you-call-it and I've never had an issue. Used regular old run-'o-the-mill paste on high end gaming PC's and the PS3 (as below). Really, I see no difference- but feel free to buy it should you want to be cautious.
Of course I haven't tried out an Xbox YET (I'm a PS3) but with the way I've been treating my PS3 slim I'm highly surprised I still have the damn thing running. I've taken it apart (I mean, ripping it apart
) then left it for 2 months on a dusty shelf, then reassembled while watching Anime (and only using 4 screws) with new thermal paste. It runs well, in fact quieter than before.