All things being equal, I find it a bit strange that while the GTS 250 draws 30w less power than the HD 4850, overall power consumption is greater. It's due to load variation on all of the components at a specific point in time.
Therefore, dropping in a GTS 250 in the same system as the HD 4850 should yield lower power consumption.
However, there is more to power consumption than just the individual components. The PSU's power efficiency has the greatest influence on total system power consumption. The less efficient the PSU is the more power will be drawn from the AC outlet to be converted into DC power. The excess will be wasted as heat and noise. Therefore, a good quality
80 Plus certified PSU is recommended.
80 Plus basically means that at any given load, the efficiency never drops below 80%. There are different levels of
80 Plus as well: Bronze, Silver, Gold. Each level means higher minimum levels of efficiency.
Below is a good resource for PSUs which I use since I look for both efficiency and low noise:
http://www.silentpcreview.com/section10.html
I tend to use Seasonic PSUs.
Seasonic S12 500w in my HTPC - From 2006 before the
80 Plus certification was introduced.
Seasonic S12II 550w in my primary PC - This is
80 Plus "Bronze" certified; Minimum 82% efficiency.