On that Via chipset, that combination of memory will probably not even boot but, the only way to know for sure is to install and test. However, even if it does boot, it probably won't be stable.
Ideally, you need to add memory that EXACTLY matches the original memory, ie-2 more identical sticks with the originals. A pair of the 1gb DIMMs might also work with the 2 x 512mb but, again, the only way to know for sure is to test.