I work for a IT company, but I am not working directly on Java. Anyway I would like to take this certification.
I am studying at home (after work) and I would like to buy the "OCA Java SE 7 Programmer I Study Guide" to improve my Java SE skills.
Do you think it would be enough for me to reach my goals?
From my understanding, the new OCA Java 7 programmer exam is a subset of the old SCJP exam. The OCP Java 7 Programmer covers the rest.
For your case it depends where you want to go. If you want to have an overview of Java platform/technology then the OLD Associate exam (OCA Java 5/6) may be a more suitable exam. However, if you really want to learn the syntax and stuff, OCA Java 7 will get you there.
To add to it... if you are brand new to Java, you'd be better off taking the OCA 5/6 exam. It's easier (based on the SCJA design)... and with the right amount of preparation, you should be able to score very well with a comfortable amount of effort. This OCA 5/6 (1Z0-850) exam is really designed for an entry level person (e.g. the associate).
Now, the OCA 7 exam is much more challenging (than the OCA 5/6) as it's based on the SCJP design. If you choose this path, I would strongly recommend that in addition to studying for the OCA 7 exam that you have at least 6 months of solid programming/development experience with Java.