MMMattt
If your Vostro 200 was shipped with a G33M02 motherboard and a 300w PSU, it will only support dual core processors.
The largest processors are supported, by the Foxconn G33M02 motherboard:
Wolfdale Core 2 Duo, E8400, 3.0GHz, 6MB, 1333FSB, 65W, E0
Wolfdale Core 2 Duo, E8500, 3.16GHz, 6MB, 1333FSB, 65W, C0
Wolfdale Core 2 Duo, E8600, 3.33GHz, 6MB, 1333FSB, 65W, E0
If your Vostro 200 MT has G45M03 motherboard and a 350w PSU, then the following Core 2 Quad CPU's are also supported,
Yorkfield Quad Core Q9400, 2.66G, 6MB, 1333FSB, R0
Yorkfield Quad Core Q9550, 2.83G, 12MB, 1333FSB, E0
Intel Core 2 Quad Q8200 (2.33 GHz, 1333 MHz FSB)
Intel Core 2 Quad Q6600 (2.4 GHz, 1066 MHz FSB)
Intel Core 2 Quad Q9300 (2.5 GHz, 1333 MHz FSB)
Intel Core 2 Quad Q9650 (3.0GHz, 1333 MHz FSB)
Prior to installing the processor, check that you have BIOS Version 1.0.16 installed, when support for new Intel CPU's was added.
More Vostro 200 MT upgrade info, here:
http://en.community.dell.com/support-forums/desktop/f/3514/p/19295125/19993592.aspx#19993592
Bev.