For games, the SMP systems are slower than single processor systems with the same cpu. The overhead required to operate the second CPU slows down the game.
For Photoshop and CAD, the dual CPU system will in many cases be faster. There are a few exceptions, however, but not enough to matter IMHO.
For games, the oxygen card is not going to be the best choice. I think overall you might be better served by using an ATI Radeon 9700 Pro which will do your 3D work well and play games VERY well.
The difference between AthlonXP and AthlonMP is in the CPU bridges. MP chips have certain bridges made/unmade which sets them up as an MP. AthlonXP chips can be modded into MP chips by reconnecting cut bridges. If this is going to be a real workstation, I'd get the real MP chips. They are more expensive but not by too much.
I hope this helps.