I have a secondhand YPT-400, which is exactly the same as the PSR-E403 except for their names. I think the YPT-400 is a lot of fun to play, so I'm not surprised that you've gotten great satisfaction from your PSR-E403.
Given your familiarity and satisfaction with the PSR-E403, the most logical suggestion might at first seem to be to get the PSR-E463 or PSR-EW410, which are very similar to the PSR-E403 but with more voices and styles, as well as twice as many registration buttons and memories, plus patterns or "grooves" (first added on the PSR-E433), a sampling feature, some DSP effects, and the ability to use a USB flash drive.
Another option-- if you don't want to spend a lot of money-- might be the PSR-S670, which has the ability to install Voice & Style Expansions.
However, if you're especially interested in Hammond organ sounds, you might want to consider a different brand. Personally, I'm very interested in the Studiologic Numa Compact 2x, although I've never seen or heard or played one in person.
This is a Yamaha-oriented forum, so I don't wish to be perceived as discouraging anyone from buying a Yamaha keyboard, but I like that the Numa Compact 2x includes a tone-wheel organ sound engine and nine drawbars, as well as a virtual synth sound engine, in addition to the usual stereo-sampled sound engine-- plus, it has a semi-weighted keyboard with 88 keys, including aftertouch, and built-in speakers.
You would probably get a better Hammond drawbar tone-wheel organ sound from the Numa Compact 2x than from Yamaha's arrangers, unless you spend a lot more money to get a Tyros or Genos, or maybe a MODX synth.
Now, if you really don't want to spend as much money as a Numa Compact 2x would cost, another option might be to upgrade to the PSR-E463 or PSR-EW410, connect it to your computer, and use the keyboard as a controller to play virtual instruments or VIs. There are a number of VIs that provide a decent Hammond simulation, and the cost of one of those programs plus a new PSR-E463 might be more in line with your desired budget. The PSR-E463 and PSR-EW410 can send and receive audio over the USB connection, so you could have the computer send the sounds generated by the VIs to the PSR-E463 keyboard so the VIs' sounds will come out of the keyboard's speakers.
