I assume that you own something that accepts no more than 4 cards like a JV1080.
The session card would be a good 1st card to buy as it fixes up many of the deficiencies in the ROM waves such as the acoustic pianos and adds 8 good drum kits for general use. The Nylon string guitars, brass and saxes are nice too as are the strings.
If you are into rock/pop, then go for the Bass&Drums card with heaps of realistic sounding bass guitar and drum kit sounds. If you are not into this and go more for dance/techno, disregard this card!!!!!!
Both the orchestral cards (1 and 11) are great for soundrack work so look at least at one of these.
I don't think too much of the techno card and I only wish Roland didn't get sued over the much better 'dance' card they had previously so it would still be available.
The Hip Hop card on the other hand is fantastic.
Well enough rambling- for what I would be doing- I would get-
SESSION.
BASS AND DRUMS.
ORCHESTRAL (whichever one!).
VINTAGE SYNTHES (fantastic).
Remember, replacing cards is very easy (unless you have your unit in a rack like myself!).
As mentioned above, it all depends on what style of music you are into. Cheers.