Upgrading from Ti 4200 (to FX 5700 LE?)

I currently have a 2.53ghz Dell 4550 and a GeForce Ti 4200 video card
with 64MB RAM. I play lots of HL and Counterstrike and it runs great.

I recently bought Far Cry and it runs great at 800x600 with medium
settings (I am running the latest nvidia drivers and DirectX 9.0b, or
whatever came with the game). I'd like to be able to crank up the
graphics a bit, which I know will require a new video card.

Unfortunately, my Dell has a rather small case and the power supply is
only 250w. This probably eliminates any of the fancy new 6800 cards.
I've been looking at the FX 5700 LE cards, though - would this be a
reasonable upgrade? Would I notice a difference? Will this card run
HL and Counterstrike well (I use the OpenGL option now and get
excellent frame rates).

Any advice appreciated.

Michael W.
Chicago, IL, USA