Well, you've got a couple of options. Probably the most fiscally minimal one would be to buy a convertible, like the My Ride you mentioned, for the upcoming new baby, and get your 3 year old a combination seat (seat that goes from harness to booster), like the Graco Nautilus.
Another option: an infant only carrier for the new baby: (Like the Graco Snugride 35 or several others), and a taller convertible for your three year old, which I'd suggest using rear facing for him. If he's pushing 40 lbs, this plan might be silly. On the other hand, if he's closer to 35 or 30, and/or if you find that a Radian works in your budget after all(which has a rear facing weight limit of 45 lbs), it might work quite nicely.
Then he stays rear facing for about a year, until baby outgrows the infant seat. You pass the Radian (or other convertible) down to the baby, and THEN you buy a combination seat.
If you think about those and let me know which you think would work better for you I can give you the names a few good seats in the appropriate categories so you can play around and check them out and figure which suits you the best.
One last (wicked important) question. For the big girl's Graco Turbobooster, *did you remember to screw the screws into the armrests* when you assembled it?
The box came with a little plastic wrapping that had the manual, a shoulder belt adjuster for backless mode, and two screws.
Where the armrests go in, where the pressure-adjusters are for the two different heights, there are two tiny predrilled holes over each adjuster. There should be a silver screw in each one.
If the screw is NOT in the armrest, the seat is not properly assembled, won't work in a crash, and needs to be fixed with new screws or replaced before your daughter rides in it again.
I ask because many, if not most, parents forget about the screws.
And just checking if you're familiar with new recommendations: Children should remain rear facing for a minimum of two years and thereafter as long as possible, children should be harnessed until they reach maturity to sit nicely without moving in a booster, and children should be boostered until they are 4'9" and fit the seatbelt safely (around 10-12 years old).