The Dimension 4200 has only PCI slots according to the Dell website.
Pentium-Ds were capable of 64-bit. However, Windows 8.1 64-bit won't load because certain instructions are not available on a Pentium D. Windows 7 and 8 64 bit will run but rather slowly because the instructions get emulated. Some Celeron D models supported 64-bit; model numbers ending in "0" are 32-bit only. Intel product marking is an awful mess. Even if you have a Celeron D with 64bit support, you would most likely be better served by something newer.
You need RAM that matches your motherboard. DDR2 would be mostly late Pentium-4 and the Core 2 era or AMD motherboards using AM2 sockets. Most of the more modern motherboards (Intel socket 2011, 1336, 1150, 1155 or AMD AM3+) will use DDR3. Check specific model of motherboard; there were some oddballs that used different memory from normal. If you want 64-bit OS, you probably will be better served with one of the later motherboards with DDR3. The chips are faster; the virtual machine instructions are better; and the memory limits are higher. Even budget systems now can handle 32 GB over 4 DDR3 slots; most DDR2 system were restricted to 8GB and DDR2 costs a lot more. With 1GB, 2 GB, and 4GB DDR3 sticks all priced about the same per stick, there is little reason to skimp on RAM.
Note that the current higher end integrated video from both Intel and AMD exceeds the capability of the HD 6450 so you might not to use a graphics card.
CPU performance of later chips is much better. Core2 is about 50% better clock for clock; Haswell is twice as good clock for clock. The under $300 Pentium boxes often seen in Best Buy will provide 2 cores, each twice as fast as what you have, while cutting power consumption by 2/3. The new Celerons are rebadged tablet chips and are quite slow and not a good choice for any gaming use.
