What size case should i get

[Axyxn]

So im  trying to change my case gor my dads hp z400 workstation which is now mine and i want to get a modern case with rgb and all i have a doubht abt what size case should i get without changing up components of my pc

[SDH]

Is it a version 1 or a version 2?  Does it have the fastest possible processor if it is a version 2?  That is, a X5690.  That is a processor never documented by HP to work on the version 2 Z400, but it does and is smoking fast.

Does it have have 48GB of 8GB RAM sticks, matched to the X5690 for best speed?  Tuned BIOS?  W10Pro64, tuned for performance?  It is running at least a 500GB SSD?  Is cooling for all this optimized with the optional front cooling fan and the optional HP memory cooling air duct system?

Size of case I'm not so sure on, but color.... must be red.

[Axyxn]

Well its proccessor is actually Intel Xeon and has 6 ram slots and the motherboards name is Intel X58 and it doesnt have ssd it has a hard disk

[SDH]

OK.... good news then.  The version 1 Z400 only had 4 memory slots.  The version 2 has 6, and much more potential.  These are "SATA II" workstations which means they don't have the SATA III technology level of the next generation (Z420, for example) but few people can tell the difference in real life.  My favorite used inexpensive SSD for these is an Intel 320 series from eBay, specifically not a HP version, 300 or 600 GB in size, and search via exactly "intel 320 300GB" or "Intel 320 600GB".  The ones from HP have their own firmware... I like to use the Intel "Toolbox" software with the Intel SSDs and that allows you to upgrade the latest Intel firmware if the drive is not yet currently at that.  It also allows some tuning and optimization of the SSD.

The processor:  Intel Xeon X5690 is excellent in these version 2 Z400s, and it has a single sSpec code of SLBVX.  That is the only processor I've chosen to use to build up a Z400 v2.  Apple used those also, and if you search eBay for SLBVX you'll see they go for roughly $100.00 USD each.  Not bad when you know that they went for $1666.00 USD each earlier.  You may get lucky by bidding on one when there is a number of them up for bid, like now.  I like to buy only from US well rated sellers, but you might choose China or elsewhere to get a lower price.  It will take some time to get to you that way.

That HP case is engineered for best cooling, by the way.  It is not red, however.