The mid cost system was done in sept 2007, it is a bit dated.
Here is my suggestion:
For gaming, start with the most important part, the vga card. 8800GTS-512-G92. You will pay a bit more than the 8800GT, but you will get a bit more performance. The real reason to get this card is for the superior cooling which exhausts the hot air out the back instead of letting it recirculate.
In a week or so, some of the the new 45nm cpu's should be available. I would look at the E8400 3.0ghz which should sell for about $200. Only if you did lots of multitasking with heavy cpu usage would I consider a quad.
I would suggest the GA-P35-DS3R. It will support 45nm cpu's. It is $30 more than the equally good DS3L, but I like the 8 usb back panel ports(vs.4) The P35 chipset now has a mature bios. There is almost no performance advantage to X38 or X45 boards if you are not into maximum overclocking.
With ram so cheap, get a 4gb kit of ddr2-800 ram. It should be <$90 after rebate. For a new build, just go with vista home premium-64 bit. It makes no sense to me to invest is XP which will not be developed further. If you have a kid who is a student, look into the substantial discounts you can get on vista.
At some time look into a tuner card. The media center application included in home premium is very good. Your pc can act as a pvr.
Find a case that appeals to you. They all will work. Personally, I don't like "bling" or cases with fussy front doors. I do like quiet. My case is the Antec Solo. The same case is also sold as the Sonata plus 550 with a good 550w psu, or as the designer 500 which is white(a good looking color) with a 500w psu.
Don't skimp on the psu. Pick a tier 1 or 2 unit from this list:
http://www.xtremesystems.org/forums/showthread.php?t=108088
A 500w or better unit from a quality vendor like PC P&C, corsair, or seasonic would be good.
You can probably pick a 500gb hard drive based on price. They are all good.
I use a samsung 203B sata burner. I like it because it is much quieter than my old and expensive plextor.
Even though you will not be overclocking, Think about getting an oem cpu cooler. Look for one with a slow turning 120mm fan that directs the hot air out the back of the case. Such a unit will be much quieter than stock.
When you look at 24" monitors, get a good one, you will be looking at it for several pc generations. In particular, look at the viewing angle. a unit with a 160 degree angle is common on cheaper units. The problem is that if you don't keep your head relatively centered, the display looks washed out. A unit with 178 degrees is better. Lower cost units may have some number of bad or stuck pixels which can be annoying. There have been NO bad pixels in the 5 samsung units I have owned.
---good luck---