Two Questions

[Crow]

1: If you are a 20th level caster and possess a +1 sword of flaming burst, and you cast Greater Magic Weapon on it, does it count as a +5 sword of flaming burst?

2: What items are there that let you mount an extra ring (besides the necklace hand-thing)? What slots do they take up in exchange?

[Dausuul]

1: If you are a 20th level caster and possess a +1 sword of flaming burst, and you cast Greater Magic Weapon on it, does it count as a +5 sword of flaming burst?

Yes.

2: What items are there that let you mount an extra ring (besides the necklace hand-thing)? What slots do they take up in exchange?

I'm not aware of any offhand.

[Drider]

It's not an item, but in eberron there's a feat that let's you use 4 rings instead of 2...