What algorithm to use for this subset sum problem
You can implement the dynamic programming algorithm using a much smaller amount of memory. You don't need the entire table in memory all the time.

The point is that to answer the question "Is there a set of items $1,\ldots,i$ that sums to $s$?", you only need the answers for $i-1$ (the previous "row" in the DP table), but you don't care about the (previously computed) answers for $i-2,i-3,\ldots$. Thus, at any given time, you'd only need an two arrays of around 120MB each in memory (and in fact, you can do it using only one array!).

However, reconstructing the solution becomes more difficult, since traditionally you'd reconstruct the solution by backtracking through all of the stored values. There's a nice workaround: with each (feasible) subproblem, store how much of the weight is taken up by the first $n/2$ elements.

We can do this computation in time $O(ns)$ (where $s$ is the sum of all elements). From the final row of the dynamic programming table, we can now find not only whether the problem has a solution at all, but also (if a solution exists), what portion of the target sum is achieved by the first $n/2$ elements (and consequently, we also know what portion of the sum the remaining elements make up). We can now solve the two subproblems recursively, each of half size. The total time taken is only $O(ns \log n)$, i.e., we only take a factor $O(\log n)$ hit in the running time for a linear reduction in memory usage.

You should also consider the possibility that perhaps not all possible values can be attained as sum of items in your set. So, depending on the input, perhaps not using a contiguous array of bits, but using instead for example a hash table will use less memory.

Finally (as an alternative solution), if you only need an approximation, consider rounding the values (e.g. to the nearest thousand) to reduce the size of the tables further.