Video card upgrade, AGP or PCI-e?

I've been trying to decide what to do with my system for a while so I'm hoping that you guys here can sway me one way or the other. My system is aging quickly but I do not have the money for a complete overhaul. I have about $200 to play with and was wondering what my best option would be.

My system
CPU: AMD 3400+
MOBO: Abit KU8
RAM: 1.5 GB PC3200
Video: ATI 9600 AIW
Sound: Audigy 2
Power: Antec Truepower 430W
Display: 19" Widescreen

I want the card mainly for gaming, I would also like to be able to game at 1440x900 resolution, I don't need all the eye candy, I really just want a playable game. OK so my real question is this, should I just stick with an AGP card for now such as a 7800GS and hold off on any other upgrades, or should I swap out the Mobo and video card and go with a 7900GS or X1900GT? On newegg I found a 7800 for $175, but they also have a X1900GT for $150, should I spend the extra for PCI-e mobo and card? A complete system upgrade isn't possible and it won't be possible for at least another year so please no suggestions to save money for that, I need something now.
Sapphire X1900GT Any thoughts on this particular card would be helpful too, anybody ever owned a Sapphire card? Right now I'm leaning towards the Sapphire card and a new mobo so if there's any suggestions for a cheaper board please let me know, I don't really need all the bells and whistles.
Thanks in advance for any suggestions and advice.

That's a high resolution you're aiming for.

If you want to stick with AGP, consider the new X1950 PRO. Hard to find in stock, but if you can find 'em they're as low as $240. Zipzoomfly carries them when they can get them.

Otherwise I'd say, go for PCIe. The 7800 GS is only as powerful as an X850 XT or 7600 GT, not really powerful enough to do 1400+ res.

I think I've decided on the X1900GT and this DFI motherboard. With the new setup should I be able to run most newer games with at least medium settings in 1440X900? Thanks again.