I am using LP-PAN with several PCs, including a 2.8 GHz Pentium 4, a two year old Dual Core laptop and a new custom built i3 based system.  Unless you plan to run just the display application at 48 kHz display width, I would recommend the following minimum PC platform...

2.8 GHz processor (the more the merrier), preferably dual or quad core.
1 GB RAM (the more the merrier, limited to 3GB on 32-bit systems, which are recommended)
16-bit sound card, preferably not integrated into motherboard. 24-bit preferred (lower noise floor)
48 kHz sound card... 96 or 192 KHz preferred (display width is roughly equal to sampling rate)
Separate video card, or integrated video with sufficient dedicated memory. Most of the SDR applications are video intensive

I highly recommend XP or Windows 7 32-bit for the operating system. I would avoid Vista or any 64-bit OS.  Almost all ham applications are 32-bit native and will run best on a 32-bit OS. Most sound cards run better on XP due to more mature drivers, but most of the recommended sound cards will run fine on Windows 7 32-bit as long as you use the ASIO driver or set up the card in Windows Mixer for MME. We have tested all the recommended sound cards and apps with Windows 7 64-bit, andthe results are spotty. Most work OK with ASIO, but not all apps support ASIO (TRX-Pan, CW Skimmer). The discontinued E-MU 0202 does not play well with 64-bit. We maintain three systems for support purposes... one with XP/SP3, one with Win7/32-bit and one with Win7/64-bit.

Note: We do not recommend more than 4GB RAM, even with a 64-bit OS, because some sound cards don't like it. You can limit the usable RAM on Windows 7 for systems with more than 4GB. This can be done in software by running MSCONFIG and selecting BOOT then Advanced Options and setting the Maximum Memory to 4095 and rebooting the computer.

Here are some recommended configurations based on the software packages and sound card you intend to run...

BASIC: LP-Bridge, TRX-Pan or NaP3, 96kHz PCI or FireWire sound card, one logger
Pentium 4 or Xeon, 2 GHz +, 1GB RAM, integrated video


MID-LEVEL: LP-Bridge, any SDR app, 192kHz PCI or FireWire sound card, CW Skimmer, multiple loggers
Dual core processor, 2.8 GHz +, 2GB RAM, separate video card (can be inexpensive card)


TOP-LEVEL: LP-Bridge, any SDR app, USB sound card, CW Skimmer, multiple loggers
Quad core processor, 4GB RAM, separate video card (can be inexpensive card)



For suggestions to improve system tuning for SDR use, check out this link...

W8FGU's paper on tuning any OS for best sound card performance. Vista Tuning for LP-PAN.pdf

Note: Do not attempt these suggestions unless you have some familiarity with computers, as some of the recommended tricks alter basic Windows operation. All tricks are reversible if you don't like the results, I believe.