Need advice on agp card

I am considering getting a newer agp card for my aging Dell dimension XPS gen 2. I really want to buy a whole new pc but as I am broke that won't happen. from what I can tell of the original specs found on the web, the power supply is below. Any concerns?

Power
DC power supply:
Wattage
460 W

Heat dissipation
1000 BTU/hr

Voltage
fixed-voltage power supply—110 V at 50/60 Hz
manual selection and auto-sensing power supplies—90 to 135 V at 50/60 Hz;
180 to 265 V at 50/60 Hz; 100 V at 50/60 Hz for Japanese computers


I was looking at these cards on newegg http://preview.tinyurl.com/6396vc My current pc has a p4 3.0ghz, 2gb ram, and a 6800gt. I am debating whether or not if it's worth it and if my power suppy can handle it and whether or not my pc will be bottlenecked. I'm not sure when I will be able to buy a new pc it may be along time and was wondering if any of these would work in my pc and make a big difference. I will be using the card for gaming and mostly play counterstrike source but I also own Call of Duty 4 and I am buying Red alert 3 which the minimum specs for Red Alert 3 are

*Processor – XP: 2.0 GHz (Intel Pentium 4; AMD Athlon 2000+; Multiple Cores) / Vista: 2.2 GHz (Intel Pentium 4/AMD Athlon 2200+/Multiple Cores)
* Video Card – NVIDIA GeForce 6800, ATI Radeon X1800 or higher end DirectX 9.0c compatible gfx card

which means my pc isn't that far above the minimum required specs.