As you have DDR2 mem, I would suggest to get a cheap mobo (~50-60 euros) and go for a 8600GTS(220 euros) or even a 8800GTS 320Mb (280 euros).
If you want to stay AGP, I would chose XFX 7950GT 512 Mb for 260 euros. Although you have also a XFX7950GT 256 Mb for 239 euros or even a XFX7900GS for 200, which is not bad for the price if you want to stay NVIDIA. If you are going to use "low" resolutions (1024x768, 1280x...) maybe you don't need the 512 Mb, although if I could I would go for it anyway...(?)
The whole think depends also if you have future plans for major upgrade or not, and when. Maybe if you are going to do a major upgrade anytime "soon" (some months, 1 year) you could get a "cheap" 7900GS for 200, and save money for later...
In my case I have 1 Gb DDR550, Prescott 3.6 GHz and Asus P5P800 (AGP). I want to upgrade in 1 year. For the moment I have got a mobo DFI 915P-T12 (DDR+DDR2) second hand and I'll get a 8800GTS new, although the CPU and RAM will bottleneck it. But for the same price that the AGP, I'll have room to OC CPU and RAM, DX10 (useful?) and eventually it will help me to avoid spend on the new PC (in a year) at same time, it will allow me to keep the gfx card some months after the major upgrade and then invest on a new one...
It took many time and headhaches to me to decide myself though...
And maybe it is not the better option...but after all, it is my option.
Just my thoughts