UPSes are roughly divided into stand-by and in-line classes. No matter what UPS you buy, it will have a surge suppressor and some basic filters.
Stand-by UPS only kicks in when the mains power becomes "bad" (too low, too high, etc.) There is a several milliseconds delay, but basic PC power supply can survive that switching delay without any problem.
In-line UPS always converts AC power to DC, and then back to AC to be fed into your equipment. So, naturally it provides better clean noise-free power, but the down side is that it is much more expensive, the double-conversion is lossy (UPS usually has a large and noisy fan to cool it down, etc.)
I've never seen in-line UPSes used outside of the realm of enterprise telecommunication equipment, where clean power is a must. I would say that a decent home UPS (they are all stand-by type) will be sufficient for your needs.
For an extra peace of mind, you can add a surge suppressor before the UPS, e.g. connect UPS to the surge suppressor, and then to the mains. However, I'd really try to obtain a decent ground, you can have lots of weird problems with your networking if your ground is bad or non-existent.