I have an Intel Mobo, an ATI TV Tuner, and an ATI Radeon X300 gfx card. I want to upgrade my gfx card, but wanted to know if it was possible, and also, which one to get that's under $250 and good gaming. Any response is appreciated  

prob the best bang for you buck on an old system like that would be an 8800gs. Good performance and a cheap price at about $109.

brands dont matter really. the limiting factors will be what type of slot you have. pciE 1.1 or 2.0 (though nothing uses 2.0 bandwidth yet) or AGP wait another week or so because there are new ATI cards coming out. Once they do you should see a drop in current prices on cards.

8800GTS 512MB?  Those are $200.  Should do you nicely.  If you need to upgrade your power supply, get an 8800GT, and a new PSU.  Those two together should run you ~$250.
 
Do you use vista?  If yes, how does the TV Tuner show up in the hardware manager?  If it shows up as a display adapter, then you need to get a 3870.  Vista requires all display adapters to use the same driver, so you can't mix and match ATI and Nvidia cards like you used to be able to.  I don't have a TV tuner, so I'm not sure how it works.

Thanks for the quick replies. I'm using XP MCE 2005, and my slots are pciE 1.1. How do I check to see if I need a new power supply?

http://www.extreme.outervision.com [...] orlite.jsp

Thanks, but I need to know what I have now.

You'd have to open it up and look at the sticker.  Don't just look at the Amperage ratings, make sure you get the totals for the combined 12V rails.  (if you have a multi rail PSU.)
 
MCE should be XP based, and you shouldn't have to worry about the one display driver at a time BS that MS introduced with Vista.

Alright, cool. Thanks for all the help