The 7300 is not really a powerful card, its less powerful than the 7600 and x1600 and they have some trouble keeping up with modern games.
I would move up to a 8600, I have one and it awesome, maxes out just about everything but the best games. The 8800gt is overrated, its an extra $150 and the 8600 actually beats it in some games like FSX.
Your cpu is a little lethargic for this kind of thing, but luckily you have a socket 775, so you can upgrade to even a quad core if you wanted.
Look at this benchmark, its comparing the 3ghz p4 to the cheap cheap 1.6ghz pentium dual core (the renamed original core duo, not the same as pentium d), the pentium 4 gets annihilated, even the 3.8ghz extreme edition p4 gets killed:
http://www23.tomshardware.com/cpu_2007.html?modelx=33&model1=911&model2=884&chart=421
The 1.6ghz os only $70 and the 2ghz is only $80, and the 2.2ghz core 2 duo is only $120, and if you went all out the quad core is only $250:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116037
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116052
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115031
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115017
With exactly the same hardware, the quad core runs 7.1 times faster than the 3ghz p4, and even the 1.6ghz PDC is twice as powerful.