I am using LP-PAN with several PCs, including an older 600 MHz Pentium Pro, my 1.3 GHz laptop and a newer 2.8 GHz Pentium 4.  Unless you plan to run just the display application at 48 kHz display width, I would recommend the following minimum PC platform...

1.8 GHz processor (the more the merrier)
512 MB RAM (the more the merrier)
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

Here are some comparative CPU usage percentages of my three systems with various configurations...

600 MHz Pentium Pro, 128 MB RAM...

PowerSDR / Soundblaster 48 kHz = 75%
PowerSDR / E-MU0202 USB(1.1) 96 kHz =100%

Toshiba Satellite laptop, 1.3GHz mobile P3, 736 MB RAM...

PowerSDR / Internal Sound Card 48 kHz = 20%
PowerSDR / E-MU0202 USB 96 kHz = 40%
PowerSDR / E-MU0202 USB 192 kHz = 65%

Dell WS360 2.8GHz P4, 512 MB RAM...

PowerSDR / M-Audio Delta192 PCI 96 kHz = 20% (card not usable for other reasons)
PowerSDR / E-MU0202 96 kHz = 40%
PowerSDR / E-MU0202 192 kHz = 60%
PowerSDR / E-MU0202 192 kHz / HRD / CWSkimmer / microHam MK2 USB audio = 70%
PowerSDR / M-Audio Firewire Audiophile 96 kHz / HRD = 10%

I you compare this machine to the laptop, it's clear that this machine needs more RAM, and I plan to expand it to 2GB, but it still works fine even with only 512 MB. It's also clear that Firewire runs more efficiently than USB.  If you use a laptop, I would consider a Firewire card in preference to USB.