The best bang for your buck will come from a "canned machine". The days of Chinese knockoff parts in these machines is pretty much over. Dells, HPs, Alienware, and more all use similar parts that you'll find at Newegg.com and other places. The exception is motherboards but honestly, I really haven't seen much difference in performance among top names any longer.
If you're intent on building one, scope
Tom's Hardware for the latest reviews of individual components. The best performance is going to come from a gaming design approach. Those are the apps that tend to tax a system the most. If it works well for gaming, it usually flies doing just about anything else. The exception is video cards. Most video cards are pretty close on 2D performance, which is what you need for PS work. There is no 3D engine requirements for photography.
Best bang for your buck on processors tends to be the 2nd through 4th from the top end. The latest/greatest is always going to be the most expensive. The next one down, through about the 4th latest, tends to decrease in cost fairly quickly. Then the curve flattens a bit and the difference between 5-whatever tends to be closer. I usually aim for #2 or #3, depending on cost/performance concerns.
Look for "dual head" video cards if you can afford it. The G550 that you're using is already a dualhead and drives two monitors with one chipset. If you can find independent chipsets, like the G450x2 (although that wouldn't be my first choice), you can calibrate each monitor individually. I don't see much of a benefit unless I'm using two considerably different types of monitors.
If you already have the OS, don't worry about packages. If you do not have a license for XP Pro (and I'll let you decide if you're really legally licensed since OEMs aren't allowed to be transferred, unbeknownst to many owners), consider getting a "barebones" package of motherboard, processor, and memory from Newegg or Tiger Direct. They'll usually include an option for the OS to go along with it.
Personally, I quit building a few years ago and have been happy with Dell's Precision line of workstations. My own Precision 390 has been handling photo work for 2 years with no issues. Adding another hard drive was as easy as popping the cover, plugging the HDD into the already installed no-tool carrier, putting the cover back on and booting up. I think it took my longer to open the packaging!
All of the features I discussed above are available in the Precisions. And they were very prompt with a replacement monitor when the backlight died on my 2407. I had it the next day.
Have fun.