800VA = 800*0.6w=480w load is better. 528w is maximum and 540w is the deadline.
I suggest, not to connect a PC with Inverter. Use your UPS with Inverter! Inverter will not regulate the AC Power. (I hope you will get a Pure SineWave inverter, if not avoid using the UPS with Inverter).
I use a servo stabilizer (For 230v AC out) -> Pure Sinewave Inverter (For Long DC to AC Backup time) -> APC 800VA UPS (Broadband Surge prot) -> PC
800VA is enough for around 500w, but buy atleast 1KVA.
I suggest you to go for Numeric Inverters, if money is not a fact for you (It cost around 20k for 1KVA). Avoid buying Su-kam or imported inverters, you can even try Microtek. Liminous is ok. No idea about Genus. I am happy with Numeric, may be the best brand in India...