With your budget, here's what I would do.

Go to any and all car stereo shops with your favorite CD in hand. Ask to hear setups with just your headunit. They will try to sell you an amp...tell them, no I don't want one, thank you very much (unless of course you do, but 400 isn't much).

I'd suggest you get the same brand/line of speakers front and rear.

Normally, I would never suggest rear speakers, but since you are fairly adamant about not having a box (keep in mind you can put one 10" sub in 0.5 cubic feet - 12" x 12" x 6") you are stuck with what the factory provides.

Find a speaker that sounds good to you, then go for it.

I would think any of the following:
BA Rally
JL Audio TR series (I think)
Polk
Infinity
Alpine

Would be a good start. No one can really tell you what to buy, you sorta have to go out there and take a look around. Try to go during lulls in the business day so that you can get some attention and get some good listening time in.

When you listen, set the bass and treble controls flat - that will level the playing field (especially if they don't have your headunit where you are shopping).

Try that route first, see what you like, then come back with more questions if you have them. Speakers are the most difficult thing to recommend over the internet.