How much ASA-5505 Firewall can helpful against virus attack?

Looks good, the inbuilt intrusion protection system looks like a good feature. if the company is paying for it I say why not. Although if you want an IDS on the cheap I recommend snort. It might be a better option to run snort as well cause there are probably more updated scan rules available for it. But asa 5505 looks liek a good (if expensive) system. I have never used it but it should provide a bit of extra security. Although you might want to look at some sort of mail filter scanner as well. ( i assume you have one on your mail server? )