I love my SG, but the thing that has always bugged me is that the sound is very treble heavy, and has virtually no sustain. Obviously an amount of that will be due to the guitar, but I was thinking of a pickup replacement to help counteract this.

It's a Epiphone G400, not sure of the original pickups but they aren't really that great. I'm after something that boosts the low end preferably, and increases the sustain if possible.

My budget isn't really high enough for something as high end as Seymour Duncans, but any suggestions are welcome
PRS SE Chris Robertson
PRS SE Angelus Custom
Yamaha SF1000 (Both of 'em)

Laney L20H Lionheart
Marshall 1936 w/ Eminence

Rather large pedalboard..
The stock pickups in that guitar are quite nice clones of the 57's Gibson uses in many of their guitars. IMO they do not need to be changed if you can adjust the height of the pickup according to your taste. If you find this to not be true take a look at GFS pick ups. They are great quality pick ups and if you choose the right set they will be an improvement on the stock pups in your G400.