Well yeah, if your a gaming person, a Athlon 64 processor would be your ticket (they are also cheaper than Pentium 4 models), though Pentiums tend to be better at muli-tasking and decoding of movies, etc (though not by a big margin. A Pentium 4 Extreme edition would also include a virtual processor for more boosts)
You are best getting a CPU model if its a Athlon 64 you want at 3000+ or more (if you find the ghz rating to be around 2.8ghz for this model, dont worry. AMD's do more in a clock than a Pentium, and so the ghz rating is not so important. 3000+ for example will mean it will perform as well as a Pentium 4 3.0ghz processor, or better in places, hence the plus)
I'd go for a AMD Athlon 64 (64bit means you will be ready for 64 bit computing when it sets off)
3000+ or 3200+ if I were you (Try and get the new Venice core, which is improved over the Winchester core for these processors, and so don't go for a older Clawhammer model which came before Winchester (basically a better manufacturing soloution allowing for a cooler pc)).
These two new forms of the Athlon 64 processor are in the form of the 939 pin which is the newest form, rather than the 754 pin on the Clawhammer models.
With the money saved by buying that speed of processor buy a good graphics card like the Geforce 6800 or 6800 Ultra if you are a heavy gamer. (CPU's don't really affect game performance that widley, though those models would surely help for other computer tasks)
Otherwise, a Geforce 6600GT if you like Nvidia cards would be suitable for all recent games at high settings and rather high resolutions, though of course a 6800 would be better equipped for better resoloutions if you like to play games higher than 1152x864 for example.
In terms of motherboard, this is much more a wide array of selections. Would you want a SLI equipped motherboard (allows you to have two graphics cards running at once for games, in PCI-E) or PCI-E, or AGP?
AGP is becoming older now, and PCI-E will allow for faster cards in the future, so its up to you
Geforce 6800 models I think can be PCI-E models if you want to keep what you have a while.
With RAM, either go for 512Mb (the standard now), or opt for 1gig of RAM in two 512mb chips if you can afford it
With the 64bit computing, and a PCI-E enabled motherboard, you would then have a long lasting PC, with only updated memory and a new graphics card every 2 years or so to have games playing at a nice res. I think you'll be fine