I need a little advice on a new build!

[MNinMA]

I am sort of new to building my own oc...so be gentle.

~ This will be for CAD, 3D modeling (I need it to support open GL), and some gaming (typically games like the sims 2)

~ I would like to keep my budget to around 600 or so...I'm pretty flexible on this if there's a legit reason...I want a pretty good machine that will last me a few years.

~ I am going to be using some parts from my current computer so I think I really only need:
- Motherboard
- Video card
- Processor

~Right now my machine is AMD and uses DDR 333 ram...but I am impartial to AMD or Intel...I don't know a lot about the differences and from what I've read in other threads, it doesn't seem to matter much.
If I did switch to Intel, would it effect or not work with any of my old components (ex: ram and harddrives) OR does it only really effect the parts I'm looking to buy?

Any help or advice would really be appreciated!!

[flanzig1]

If you go with a fairly up to date motherboard and CPU; you will probably will need to upgrade the RAM also. You will be able to reuse opticals and HD/floppy drives.
If you reusing the same case, probably will need a new PSU as well.

[jayb1234]

You will need a new copy of Windows too.

[glc]

You may or may not need a new copy of Windows, depending on exactly what you have now. The reason you may need a new PSU is if you get a PCI-E board (recommended) you need an ATX 2.0 spec PSU with a 24 pin or 20+4 pin main connector. DDR333 ram *will* work on a new build, but will be holding you back if you use a 200/800 FSB processor. For CAD and 3D modeling, I'd put at least 1gb in. Games like the Sims don't need a particularly powerful video card, but you should get one with 256mb ram (this will help with your CAD and 3D modeling) and *not* a hypermemory or turbocache model. Something in the $100 range should be plenty good.