If 640x512 isn't high enough resolution isn't high enough for you you can boost the resolution horizontally to 1280x512 interlaced but it will be slow in super-highres mode.
Vertically will be the big challange. You can use overscan to get a few more pixels vertically but I doubt a PAL display will go over 560 and still be completely visible.
If you have a "silver-box" adapter for the AGA chipset then you'll be able to connect to a VGA monitor and, depending on the monitor you choose, be able to get 800x600 super-high resolution.
The moments that a "silver-box" adapter will come in handy will be rare if you play a lot of games so a scan-doubler would be your next option and there are only a few available.