Current OEM PSU Model: NPS-250CB/ATX250-3505 (115/230V); dimensions
5.9"(W) x 5.5"(D) x 3.4"(H).  Only original h/w specs change is a new
128MB Radeon 9800 Pro AGP, which recommends at least 300W PSU.

Please identify an upgrade PSU that meets at least new min. power
supply needs, but would prefer maybe 400W (for moderate future system
power needs).  NOTE:  Dimensions and system compatability must be
exact fit, and would prefer PSU from reputable on-line provider with
total cost of less than 80USD (or at most 100USD).  I like the fact
that 80mm fans with air flow seem to have best specs based on my own
google search research, but really would like confirmation, so I am
not buying an inferior PSU, and really need exact dimension fit due to
tower case design. I have seen some dimensions shown as L x W x H, and
not sure if this is the same exact match to W x D x H.

That's a standard sized power supply...just about any one you purchase
should fit.  I like powersupplies by antec.  They consistently get
good reviews for power output and system stability.

I recommend getting one of their truepower series supplies.
http://www.antec.com/us/productDetails.php?ProdID=22380

You can order from their online store, or I recommend ordering from www.newegg.com

http://www.resellerratings.com/seller2121.html
http://www.newegg.com