Although I am not buying a new gaming pc for Second Life today, I am looking at deals.
Here is a very good buy over at Newegg for $570.00:
iBUYPOWER Gamer Extreme NE721i Desktop PC
Intel Core i5 3350P(3.10GHz)
Note: this is not the best Intel chip set around, but it is more than adequate for most Second Life needs. I am not confident that this would provide enough power to someone who needed to do advanced multimedia work such as high end machinima production, for example.
8GB DDR3 500GB HDD
Capacity NVIDIA GeForce GTX 650 1GB
This is a very good deal.
The NVDIA graphics card is particularly good and sells for about $130.00. It would be very hard to put this together from parts at this price. Remember -- you need a monitor with this computer. They have become very inexpensive though, a nice 23 inch monitor by Acer is on sale at New Egg for $109.00 today.