OK, the UPS has a capacity of 650 VA. Then that is the TOTAL power available from it for ALL items plugged into it. It does not matter how the connections are made. So if you use extension cords and a multiple-outlet power bar to connect five devices to the UPS, the TOTAL power available to all of them is still 650 VA max. With UPS's it seems (I'm not sure I understand fully) that the WATTS max is a bit smaller than the VA max.
Now, you say the computer consumes 400 Watts max. Based on what? Did you actually measure its power consumption? Or, is that the rating of the PSU module in it? If that is the PSU rating, then two factors throw that into doubt. One is that a PSU's RATING is the max it can OUTPUT to your computer. Its actual consumption from the wall outlet is more, depending on the PSU's efficiency. If that is 75%, for example, then the PSU could consume up to 500 W from the wall at full load. The second factor, in the opposite direction, is that most computers are designed so that they actually consume LESS than the PSU can provide. So even if the PSU is rated at 400 W and MIGHT consume 500 W at full load, it is very likely that the real consumption from the wall outlet is more like 300 W or less. But you really cannot know that without measuring it under heavily-loaded conditions.
If your main interest is Voltage regulation and surge protection, I can see why you would want as many of your devices as possible to be run off this device - computer, monitor, printer, network router, internet modem, etc. How long the system could keep working through a power failure is of much less importance. So somehow you need to get a good handle on that total load. If you could buy or borrow a wattmeter, (or maybe an AC amp meter) you could plug all your deices into a power bar, then plug the power bar into a wall outlet through the wattmeter, and arrange some heavy use of everything all at once. That would measure your maximum Watts consumption. You would need to do this only once to get a reasonable estimate. If you don't understand electricity and how to do this yourself, you'll need some help - maybe hire an electrical tradesman for a short job. That person may supply the require measurement tools.