Question : Problem: 375W + Dell + 9800

How high can i go with

Dell 375 W (30A)
Pentium D 3GHz
2GB RAM

Can i go for the 9800 (or how high can i go?)

Answer : Problem: 375W + Dell + 9800

The limit will be the single PCI Express slot you have, so basically eliminates SLI where dual PCI Express is needed.
So a 9800 is fine.
Its just the power you'll need to work out.
http://www.xbitlabs.com/articles/video/display/ati-powercons_8.html
you could try the lite version of this:http://extreme.outervision.com/psucalculator.jsp
of this web based calc:http://www.thermaltake.outervision.com/
I think the power supply should be ok, however really depends on the USB devices, Hard drives, Fans and optional hardware...
There is a power monitor available around, I use probeII.
http://www.dell.com/html/us/products/optiplex/calculator/index.html