Actually, the renderer does matter, but since you're on XP, you're right, there's nothing you can do in terms of if you can use DX9, 10, or 11. The videocard and the OS both have to support DX10 or DX11, and for that you need to go Vista (DX10) or Win7 (DX11) and have a videocard compatible with those capabilities, to boot.
The long and short of why DX10 is better than DX9 for PCSX2 has to do with how things work between DX9 and DX10. A lot of things that are easier or quicker to do in DX10 are hard or impossible to do the same way in DX9, for example, 32-bit depth buffers, to avoid Z-Fighting. This was only - finally - added to the DX9 renderer in r3002. The DX10/11 renderer has had such a thing for a long, long time, because it was easier to implement on DX10 - in DX9, basically, lots of nasty little tricks have to be pulled to get it to do what they need it to do.
There's also the fact that a DX10-class card is relatively cheap now (Easily under $100 US), Vista/Win7 have been out for 3-4 years, and most of the issues that used to plague it have been moot for several years now. DX9, for all intents and purposes, is outdated, and more and more devs are taking advantage of the DX10/11 stuff.
It will still work with Melty, but that's because Melty is hardly a complex game. Even then, it's still possibly slower or inferior to the DX10/11 renderers.
Keep in mind that even though the game is a simple, 2D game, you are still emulating a whole PS2, down to the hardware. And simply put, DX9 can't do the video rendering part of that nearly as good as DX10/11 can.
That said, you COULD in theory upgrade the CPU to allow you to use the SSSE3 or SSE4 renderers... SSSE3 would be basically any Intel Core 2, SSE4 will be found in the 45nm Core 2s. Any i-series (i5/i7) will have SSE4 by default, as well. This would give a small speedup, but obviously, don't go spending several hundreds of dollars if it already runs acceptably for you.
