the xbox ( pretty much the same as a pentium 3 pc) that should give you some ideas. only actual difference between the xbox and pc is the xbox uses a striped down version of windows2000 on a FatX partition.
Xbox Tech Specs:
* CPU: 32-bit 733 MHz Custom Intel Coppermine-based processor in a Micro-PGA2 package. 180 nm process. (Equivulant to a Pentium 3
o SSE floating point SIMD. Four single-precision floating point numbers per clock cycle.
o MMX integer SIMD
o 133 MHz 64-bit GTL+ front side bus to GPU
o 32 KB L1 cache. 128 KB on-die L2 "Advanced Transfer Cache"
* Shared memory subsystem
o 64 MB DDR SDRAM at 200 MHz; 3.2 GB/s
o Supplied by Hynix or Samsung depending on manufacture date and location
* GPU and system chipset: 233 MHz "NV2A" ASIC. Co-developed by Microsoft and NVIDIA.
o Geometry engine: 115 million vertices/second, 125 million particles/second (peak)
o 4 pixel pipelines with 2 texture units each
o 932 megapixels/second (233 MHz x 4 pipelines), 1,864 megatexels/second (932 MP×2 texture units) (peak)
+ Peak triangle performance (32pixel divided from filrate): 29,125,000 32-pixel triangles/s raw or w. 2 textures and lit.
# 485,416 triangles per frame at 60 frame/s
# 970,833 triangles per frame at 30 frame/s
o 8 textures per pass, texture compression, full scene anti-aliasing (NV Quincunx, supersampling, multisampling)
o Bilinear, trilinear, and anisotropic texture filtering
o Similar to the GeForce 3 and GeForce 4 PC GPUs
* Storage media
o 2×–5× (2.6 MB/s–6.6 MB/s) CAV DVD-ROM
o 8 or 10 GB, 3.5 in, 5,400 RPM hard disk. Formatted to 8 GB. FATX file system.
o Optional 8 MB memory card for saved game file transfer.
* Audio processor: NVIDIA "MCPX" (a.k.a. SoundStorm "NVAPU")
o 64 3D sound channels (up to 256 stereo voices)
o HRTF Sensaura 3D enhancement
o MIDI DLS2 Support
o Monaural, Stereo, Dolby Surround, Dolby Digital Live 5.1, and DTS Surround (DVD movies only) audio output options
* Integrated 10/100BASE-TX wired ethernet
* DVD movie playback
* A/V outputs: composite video, S-Video, component video, SCART, Digital Optical TOSLINK, and stereo RCA analog audio
* Resolutions: 480i, 480p, 576i, 720p, 1080i
* Controller ports: 4 proprietary USB 1.1 ports
* Weight: 3.86 kg (8.5 lb)
Also the 360 just uses a a mac cpu. but I'm guessing it will depend on which cpu style you perfer. If your going to build a custom consel why not try using a amd phenom ll 4x. AMD is better for gaming plus if cooled right you can overclock it to 6.3Ghz per core stable. then again intel is better for multimedia. So hope these give you a better idea.