Hardware RAID generally requires drives of equal size to build the array. If you partition a portion of one drive for your OS, outside the array, you limit the size of your RAID because the other drives can be no larger than the smallest drive in the array. You should have a small separate drive, like an SSD, dedicated to your OS and apps that is not part of the RAID array. Win-7 does not have a 2TB limit but you are using a BIOS RAID function on an old computer which will be dated and so it is not surprising that it has the 2TB limit of MFT partitions.
There are several different approaches I can suggest, none of which use the dated BIOS RAID function of your Dell. The easiest, cheapest and most flexible would be a software RAID solution like FlexRaid to run under Win-7. You can put in 4 drives of any size, reserve a partition on the C: drive for the OS and apps and combine the rest into a RAID array with single or multi-disk redundancy. At a later time you can add additional HDD's via external enclosure and include them in the original array or create a second array.
If you prefer to use hardware RAID, you can buy an external 4 or 8-bay RAID enclosure that will attach to your Dell via eSATA or USB -- i.e. Mediasonic ProRAID 4 Bay Enclosure (HFR2-SU3S2) or the Mediasonic ProRAID 8 Bay Enclosure (H8R2-SU3S2). Or you can choose to buy a PCIe RAID card and non-RAID 4-bay or 8-bay enclosures (i.e. Mediasonic HF2-SU3S2 or H82-SU3S2). Hardware RAID solutions like these will require all drives be the same size and if you start with a 3 drive RAID 5 array and later want to add a fourth HDD, you will generally have to rebuild the array from scratch. The upfront cost of buying all the drives at once can be substantial and you have to put some thought into how much storage you will need for the future so you don't buy undersized drives.