It's not that it's a bad thing. But it could be better. You will see a big difference in detail if you set the resolution higher. It's a matter of taste and standard of content really, but if you can go higher, why use the lower resolution? A 42 inch sholud be capable of at least HD ready resolution. I even set it on my 19 inch monitor at this very moment.
And most of all, the specs for the new PC will be a waste of, well, everything if you use such low resolution. It's up to you of course, but if it's not efficient, a PC build is not a good build. In your case, it's an overkill. At least use the native resolution of your TV, so that you can get the optimum graphic output.