When buying a chip for the GC you need to look at the group behind it... unlike the Xbox the bioses are generally chip specific. There are SOME universal bioses but for the most part you'll want to use the bios specific to your chip.
With that said reading up on bios bugs, reliability, frequency of release, etc. I decided to get a Qoob for myself.
If you're into homebrew stuff, action replay, etc. then get a Qoob Pro hands down it will be the best chip for you. If you mostly just want a cheap way to play backups then I'd recommend the Qoob SX... I picked it up for $32 from the console corner.
The Viper is a good chip too but it's got the price tag of the Qoob Pro with the features of the Qoob SX...
A good resource is the Mod That Cube FAQ
... it's got comparison charts for bioses and chips. Also take a look at the install guides... the Qoob and the Viper are the two easiest chips to install.
Also I'd recommend getting a BroadBand Adapter... you can get buy without one but if you want to rip and games or load homebrew apps from over the network you'll need one.
Also I should note that most chips ship with the ability to read DVDRs disabled to comply with the DMCA which means if a chips only 2 methods of flashing are DVD and BBA, the initial flash can only be done by the BBA (as was the case with my qoob sx)...
Also the bolts on the outside of the case are special game bits, if you don't have the right bit in your toolbox (most people don't) then you'll need to buy one, Alone they can run you anywhere from $4 to $9 + S&H plus you'll need a driver to actually use the bit. I happened to already have the right bits as they came in a set I bought years ago as part of an arcade repair kit (best $14 I ever spent) thought I've yet to find a set for sale online that includes the GC bolts with it.
Just some things to consider before diving into GC modding...
I'd strongly recommend reading through the Mod That Cube FAQ