Nice to see some shmup love on here. I, too, specifically designed my bartop to be a shmup machine, but in a way so that I could rotate the monitor to hori. Couldn't agree more with Emphatic, it's worth it just for the old school stuff and for Progear. Plus, there's Akai Katana coming up in a year or two... just a couple more thoughts:
- If you're not entirely sure there's no way anything else but shmups will be played on your machine, just use a 6-button-layout. Might be you want to play a game of Street Fighter/ BlazBlue/ whatever some time, and it's not really that much more effort to install 6 buttons instead of 4. Besides, you will be able to play around with button configuration and will be more flexible.
- A 2 player panel doesn't make that much sense if you rarely play two player games; even more important, if you mainly play alone, you will sit slightly to the left or right of the centre of the screen which will give you a stiff neck after more than an hour playtime. Simply add a usb plug under the cp so if you really want to play with a buddy he/ she can plug in a USB joystick. Hell, add 3 usb plugs and you will be able to play sidescrolling beat em ups with three other dudes.
- Consider getting a true 4:3 screen; not only do 4:3 games look a tad ---smurfy--- on 16:9 screens, but for vertical shmups, movement of your ship will be slightly screwed up (up- and downward movements will be quicker than left right movements). Remember that precision is incredibly important in shmups. You should be able to still get a good 20" LCD screen for an ok price (got mine for around 100 EUros on Ebay; it was used, but in great condition. Hasn't failed me yet.)