1- RMA is Returmn Merchandise Agreement, returning the unused item. it always helps if the seller has what you want in exchange.
2- You have to shop for CPUs by SLxxx "S spec." to get the older C0 stepping. The CPUID number changed after that and they won't work
from fastest to good enough are
X5460 SLANP 3.15GHz makes this one of the fastest Optis. The 120W CPU is rare in these. It requires a Xeon adapter, and maybe BIOS microcode update.
Go here for info. on this. http://www.delidded.com/ You might save this for later.
Q9550 SLAN4 Most simple and cost effective CPU. The faster Q9650 is E0 stepping and won't work.
That was an interesting research project. The usual reliable sources were wrong, and nobody had done all the good mods to the same machine.
3-The memory question is the most important. Without 8GB RAM the GTX750 1GB is the best you can do. Once you upgrade the RAM you can get the GTX1050 (Ti) 2/4GB GPUs. If you have to mod the GTX750 you won't be able to return it. If it's a full height 2 slot card it will need the bracket cut in 1/2. if it's a Low profile version that can run full height single slot then it's OK. The GTX 1050 is huge news for these computers, it's a full 1080P GPU that runs off of the PCIe slot. if you install the 750, then install the 8GB RAM then you will need to buy the GTX1050 also to benefit from it. It would be better to buy the RAM, and the GTX1050 in the first place.
4- The Zotac GTX1050(Ti) Mini has just one expansion slot bracket. The cooler is 2 slots wide but that doesn't matter because it's also very short in length. So you don't need to cut the bracket in 1/2. Some cards are long enough they need mods at the front. Mostly high power3ed ITX cards.
https://www.neweggbusiness.com/Product/Product.aspx?Item=9B-14-500-412&nm_mc=KNC-GoogleBiz-PC&cm_mmc=KNC-GoogleBiz-PC-_-pla-_-Video+Card+-+Nvidia-_-9B-14-500-412&gclid=EAIaIQobChMI5KTD3Ku81wIVkoF-Ch2SiAA9EAkYAyABEgImcfD_BwE
i'm going to post a link to the Optiplex 780 page so you can see some computers running this CPU/GPU combo. Your scores will be a tick lower due to the 780s DDR3 RAM. But they can't run that 120W XEON ever.
http://www.userbenchmark.com/System/Dell-OptiPlex-780/3883
I would do the RAM first, then the CPU. and get the Zotac GTX1050 Mini. The 1050TI. or even the GTX1060 3GB mini with your PSU would be higher cost options.
The 1060 has hard mounted DVI port in the 2nd bracket, so a case mod would be required to use it.
The Xeon would be a separate project, but also the cherry on top!