what to do to my car?

Really depends on your trunk. 12s in the right box will do better than 15s in the wrong box. Go ported if you want loud bass. Go sealed if you want smaller boxes and tighter more sq oriented bass suggestions in the $200 range per sub new include sa subs, xfl subs ,obsidians dc lvl 2 ascendant arsenals tantric md or ld pick your poison

how much do you really want to spend... if your going over 1.5k rms your gonna need electrical upgrades including big 3 and AGM batteries. If your going over 2500 rms your gonna need better mids and highs to keep up with your system and also a high output alternator and extra batteries...

All advice aside, to answer your original question SoundQubed HDS315

I'm guessing he's keeping the 500 wrms amp he's got, considering he didn't mention a change in that.

However, I would take the $400 you have available, and get ONE 15 and a cheap amp that'll put out some decent power. I'd say run a BRX1100.1 on a SA15 and call it a day. I'm sure it'll sound way better and be much louder than the P3s or whatever you've got now. But you'd also wanna spend another $60-70 and buy the materials to build a proper ported enclosure.