preamps can cost from a few hundred ringgit to a few thousand. u can try cheap tube mic preamps such as behringer mic100, or one by dbx. i've tried these two budget tube preamps. i have also tried(and work with them for live purpose) lots of different high end preamps and u'll be surprised how much it will change your tone. it wont change it in such a drastic way as a guitar amp does but the difference can be heard(eg: a more enhanced sound).

recording using a laptop isnt much of a problem if u know how to set things up properly. i did a number of recordings for ppl using my laptop and still it sounded good. cant upload any now because my uni's internet is down and i cant afford to go to starbucks just to use their wifi.

pix, preamp plays more role in moulding the tone, not amplifying it. that will be the poweramp's role.

leadaxer, if u want a room ambience or a studio feel to your recordings, u can always add studio reverb, where u can adjust everything from room size, ceiling height, amount of reverb, etc. Remember after recording your guitar, there are still lots to do to make it sound good such as compress, eq, etc.

i've never tried software guitar amp modelling before so i wouldnt know. i've used behringer, podxt, boss gt8 and others. theyre very good. having good equipments is one thing, knowing how to use them to get the best out of them is another.

if i'm not mistaken, those onboard PC soundcards have preamps in their mic jacks. So if u use another preamp ur going to get distortion.

My suggestion would be to get another one of those cheap desktop mics for recording. So you have 1 mic for live and 1 mic for recording. The quality is not that bad considering it's only speech. Simple as it may sound, i think this is the easiest way to do it.

Erm.. I'm not a sifu... =)

But anyway.. I'm not sure I understand correctly.

You already have a PA system setup with your amperes? And it has a unbalanced out? What I would try doing is just make sure your 20dB boost is disabled on your mic input.. and just plug it in and see what it sounds like. At the worst.. it'll be distorted. (I just tested - just using a standard line-level unbalanced cable run to the mic input, 20dB boost off, and make sure your signal from your PA system is turned very low, and it should be fine)

Example how "bad" it can be:

http://www.graceabundant.com/other/micinput.mp3

That's headphone output of one laptop playing music, connected to the mic input of my Dell Inspiron 600m, headphone output about 10% volume, 20dB boost off. No resistor =)

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Anyway, if you are unable to do that and only have the PC mic, you have 2 options, I'd say

1) Put it right in front of the PA speaker. You'll get pretty bad sound quality, but it should have the speaker isolated
2) Put it as near to the speaker (the person) as possible. The further away it is, the more room sound you're going to pick up.

If the speaker is already using a mic connected to the PA system.. then just directly connect the PA system to your laptop as mentioned above. But please don't blame me if your laptop blows.. =\ Hasn't blown for me yet..