Hi!
I really don't know how to answer this question quickly without generating any confusion and a ton of questions answers to which you will undoubtably find by searching archives and posting more topics.
To make a long story short...
You do have to start somwhere. Since you seem to be after professional quality studio recording you will need a decent quality audio card.
Once you've got the card you will probably want to get a good digital audio/MIDI sequencer software. You might or might not need to upgrade your computer for that. It all depends on what you have. If it's processor speed, RAM and available hard drive space meet the system requirements of the software that you will decide to use then you should be okay. FOr a while. If not you wil need tto invest in either upgrading your existing computer or just throwing it away and geting a new one.
Getting an additional synthesizer workstation is not so much a necessity but an option that you might want to explore anyway. Given the power of software synthesizers having hardware synths is not really a necessity. Some get away with just using a keyboard controller with no sounds of it's own and a whole arsenal of software instruments. Some prefer mixing hardware synths with software. Some hate mouse clicking altogether and stick to using only hardware digital recorders and synthesizers. It is all a matter of preference.
So to summarise, I only have to say a few things. FIrst, explore and google everything that I've mentioned. Search this forum too. You will find that many questions that you might already have in mind have been answered by many wise ones.
Also, post in either The Bar or General Arranger keyboard sections of SZ. Traffic there is a bit better and you will get more answers. Most of us own professional recording studios so feel free to ask about anything (unless of coarse you first find the answer in archives).
Good luck.
-ED-
[This message has been edited by 3351 (edited 08-23-2005).]