I've been trying to figure how to set up a good custom PC for gaming and 3D rendering recently, so I'd like to share with you the results of my investigations. Among other sites, I've trusted the advice given at the configspc.com (french speaking) forum, at cgtalk.com and these benchmarks:

http://www.cpubenchmark.net/high_end_cpus.html

http://www.videocardbenchmark.net

So, here is a good config that you can find for a reasonable price. It's not cheap either, but it's powerful stuff :)

CPU:

INTEL Core 2 Quad 2.66GHz Q9400 + air vent [ LGA775 - 6MB - 45nm - 1333MHz]

NVidia Video card:

EVGA GeForce GTX 260 Core 216 - 55nm PCIe ( GeForce GTX260 896MB 2xDVI HDTV )

You can also get the corresponding ATI card (approximately same price and efficiency):

 HIS HD 4870 Fan ( Radeon HD4870 1024MB 2xDVI TV )

Make sure it's the 1Gb version.

Motherboard:

ASUS P5Q SE ( Intel P45/ICH10 - Socket 775 - FSB 1600 )

(this motherboard doesn't allow to have two graphic cards, and it doesn't have firewire and isn't good enough for overclocking)

RAM:

DDR2 4GB [2GBx2] DDR800 (PC2-6400) - CORSAIR [TWIN2X4096-6400C5 G]

Case:

THERMALTAKE V9 (Black) ( 4 x 5.25 ) - No Power [VJ40001W2Z]

Power supply:

650W CORSAIR TX Series [CMPSU-650TXEU]

Hard drive:

SATA300 500GB - 7200 SEAGATE Barracuda 7200.12 (16MB) [ST3500418AS]

If you want to reduce the price a bit, you can take a Q6600 processor, which is another quad core CPU. From what I've read, 3D rendering is one of the applications that really use these. The GPU (video card) is useful for previewing in 3D applications, and of course for games! The RAM helps loading big textures for example. The big power supply is intended to provide enough energy when the GPU will be upgraded in 2 years approximately, and if you need to add other things in your PC.

This is a config made in May, 2009.

 EDIT.: I've bought this config and it rocks!!! When rendering in Carrara 7 pro, it's like 10 times faster than my laptop ;) I now have a whole new world of possibilities in front of me, LOL! By the way, I'm not affiliated with the companies that make these products.