I know this is kinda noob question but I cant find satisfied answers thru research.

How much difference can preamp (all at reasonable price) make? at entry level compare to receiver, mid or high end?

My original knowledge is to use mini->2 RCA output from my external USB soundcard connect to receiver or an amp then to the bookshelf speakers.

Then I have heard some suggestions for going an intergrated amp. Why do I need a preamp? How much difference it will make?

Im planning to get LSI7 or Paradigm Studio 10/20 for my computer. I need something to power them but preamp wasnt in my head. What is ur guys suggestion?

I was thinking the following options
1. $200 used amp for bookselfs (rated 100wpc like nad) and thats it
2. $200 used intergrated amp with lower power lets say very old marantz (cause Ive seen many ppl selling them)
3. buy a $200 used amp and buy a preamp in the future (if its gonna make a significant difference)