hi,
Depending on the sound quality you want,
you'll need
a good sound card like. A delta 44 has got rave reviews and cost $230. it is 24 bit/96 khz, so you get the best sound quality
A sound module or two.
A jv 1010 is awesome awesome for what you want to do. It costs about $300 on ebay.
You have a fast computer, so you can buy gigasampler Le for $199. Some nice sample cds can be found on the internet. Depending on quality, you might not even need gigasampler. THe jv1010 might be all you need.
you need a keyboard WITH midi, to control either your sound module or your sampler. A basic radio shack keyboard with 61 keys will do just fine. As a bonus, the keyboard also has some sounds that you can use in addition to your sound module.
A software audio/midi sequencer.
Logic, DP, cubase etc. THe top of the line versions cost approx $500, but logic audio gold is about $300
Some nice monitors, so that you can hear exactly very accurately.
The famous yorkvilles will only set you back about $100 a piece. the ysm1 model. Make sure you don't try to mix with headphones. It will sound very mushy when played back on a regular system.
That's about it.
peace man