To photograph artwork, you'll need as flat and uniform a lighting setup as possible. Ideally, four lights from each corner to minimize any variation. You should use an incident meter to verify that the the light varies by no more than 1/4 stop across the artwork.

You don't need to use softboxes - bare bulbs are sufficient, if placed far enough away to minimize falloff. Minor differences due to age and color of softboxes can result in varying color temperature across your scene. This is less likely with bare bulbs. Make sure that the flash tubes are not too different - you can test this by shooting images of a grey card illuminated with each light, in turn. You should have a white balance temperature variation no greater than 150-200K, 100K if you're lucky.

Once you have the lighting set up, use a Gretag Macbeth color checker to obtain a reference color image, which you can use to profile your camera for the specific lighting setup.

Export images to 16-bit TIFF and Adobe 1998 color space - this should be more than enough for clients.