I suggest you install a X64 based OS.
If you like to play games, it is better for the graphics card to read a 64-bit phrase.
Some other advantages are the full usage of 4gb, and optimum hardware performance as in graphics.
In reference to install programs, most of them work on X64 OS. You can even hardly notice it. But there are very few programs as regcure or other security programs for websites that won't work on X64.
There are currently no drivers for HP Photosmart 1315 Printer for X64, however there are for HP PSC 1315 All-in-One Printer in either for Vista or XP64. You can check your drivers at Hp by choosing star detection:
http://www.hp.com/#Support
For your OKI C5150n laser printer, there are currently no specific drivers made for XP64 or Vista X64. But, you can use the XP drivers, which will also work on X64.
You can download your drivers here:
http://my.okidata.com/PP-C5150n.nsf?opendatabase
You should have no problems with your Radeon 4780 graphics card. You can download drivers for any OS here:
http://ati.amd.com/support/driver.html
Regards.
Alsr71