The links posted are to products that would expand arrangers/organs up to a complete new level

Garritan: A good way to get started Garritan

Steinberg: Sounds and Effects Steinberg VST

Native Instruments: The Komplete bundles being the best value Bundles look at Sampled & Acoustic + Sound & Design for inspiration.

East West: Moving up in cost, however they really are top notch East West and they also come in bundles.

VSL: For orchestral work these are probably the best there is VSL but it can get expensive. (A number of the sounds also feature in Native Instruments Kontakt)

In addition to packages, there are many VST dedicated to just 1 type of instrument. (A number of links can be found below)

Piano: Pianoteq a sound modelling piano, Ivory possibly the best sampled pianos out there.

Guitar: Real Guitar is probably as good as it gets for acoustic Real Guitar they also do a good Stratocaster Real Strat

Organ Electronic: VB3 for a top notch Hammond and Vintage Organs Vintage Organs (which is included in the Native Instrument Komplete bundles) which as well as Hammond includes other popular organs.

Organ Classic/Theatre: Pipe Organ is probably the best there is at the moment.

Drums: Toontracks EzDrummer is good to get you going, however popular is BFD BFD

There are of course many others which a quick google search will find.

If you compare the prices of add on packages for arrangers/organs, you will soon come to the conclusion that they are very poor value compared to VST, as the VST gives you far more sounds and features, plus a quality of sound that leaves any current arranger/organ on the market sounding pretty shabby.

Hope this peaks your interest

Bill