Hi there,
There are a few things you need to consider:
Number of I/O
Latency
Connectivity
Quality
Software
First, you need to be aware that not all jack inputs/outputs are the same. Guitar output is hi Z (impedance), low level and thus you need either a soundcard with will accept (or has an option for) hi Z input. Inputs on semi-pro (read home recording to budget studio) audio interfaces will usually accept Mic (usually on XLR) and line (usually TS or TRS 1/4" Jack) inputs. Some have an option to switch an input to Hi Z to plug a guitar directly in. The other option is to purchase a DI box. A DI box takes mic/guitar level signals and converts them to line level with the correct impedance.
Latency is a major issue with regards to recording. Hi latency can be extremely distracting when playing and monitoring back at the same time. Latency is effectively caused by the A/D and D/A processes involved in recording and listening back on a computers. Long story short it's to do with buffers and smaller buffers reduce the latency. However, smaller buffers reqire higher performance from the computer as the effectively allow less margin for error. When recording a single audio steam this is well within even 2-3year old computer territory.
A lot of soundcards now provide a direct monitor function where you don't actually hear back what is going through the computer. Instead the input is routed both into the computer and directly to the output at the same time so you hear an instant performance. However, this prevents you from hearing any effects like compression or reverb that are being added by the computer whilst recording (although when listening back to the recording after the performance you can hear the effects).
From your post it sounds like it might be hand for the device to be portable and I'm pretty sure that some guitar effects pedals can not double up as an audio interface for direct connection to a PC via USB2 or firewire.
The final thing to consider is how you are going to record with the PC. Names like Cubase and Protools get thrown around far too easily. For your purposes, a freeware multitracker will be fine. I'm sure there are ones that worth with VST plugins which allow for a good variety of effects to be applied. There are 1000s of free VST plugins floating around. As for hte multitracker, the name Audacity comes to mind but I can't remember off hand. Just found it http://audacity.sourceforge.net/
It is probably worth looking at www.soundonsound.com forum. The guys on there know just about everything there is to do with recording and computers. Also, not for price but for a good breakdown of what is on the market look at turnkey.co.uk. They stock just about everything so it's easy to compare features of different devices.
Hope this helps.