I am wondering if this GTX 570 will work and boost performance or if I can or should look at other spots in my system to improve. Specifics are helpfull as I am new to all of this. I mostly play games on this machine like DCUO and RIFT. Thank you for any help
Yes it'll be a nice upgrade. You might also consider 8 gigs of RAM (or even 6 gigs with two 1gb sticks) to allow Windows to store more info on the RAM and less in file paging, which will help speed up everything. Also an SSD for the OS is a very good upgrade for overal system speedyness. You won't see much difference in games unless you put them on the SSD, in which case load times will be dramatically reduced. You could also look at a new motherboard to overclock with, but keep the rest the same. Also I don't know what OS you use, but Windows 7 64bit would be a great upgrade of XP. If you have Vista 64 then it's not a big upgrade.
A word of caution, you might have to upgrade your PSU for the 570.